The same reason "collect it all" is now public, political leaders talk of tracking all communications and new encryption will have backdoors and trapdoors as offered or sold.
Bureaucrats, technocrats, contractors and pundits understand that every aspect of the internet is trackable, all encryption use can be traced and decoded.
Political leaders have often talked about material in public to sell a story. Quoting from decrypted embassy material over the decades to the the new policy statements about tracking all communications.
With all the new public statements about internet tracking why stop now?

Most other nations will still have staff on site for a city, state, province or vital sector of their infrastructure.
A huge coal supply, cooling water and the staff can keep the lights on if the nation is ready and fully understands its own internal networks. Teams can work on error messages induced by national networks or just focus on the networking they can support.
Most nations should have kept the internet, a companies external email and billing networks away from critical infrastructure.
If a company did not keep that aspect air gapped then any code can find its way in from any email or connection request.
The real loss will be in big brands marketing. Who would trust a total national upgrade from strange foreign brands during bidding?
Bids by many nations will be rejected early on the question of country of origin and security.
40 or 100 years of engineering excellence and competitive prices will not even be considered. Another nation or a local cartel might be the only systems considered. The trust is gone.

The rest of the world can just have staff and teams drive out to the more remote sites and watch systems as was done years ago.
If a dedicated hardened network requests random fault inducing commands real staff on site can make calls at 4am.
Just as other nations can revert to the typewriter and one time pad staff can revert to systems that worked over generations.
Networks are great for tracking vast systems but local vetted staff can be trusted with the more vital network wide vital commands.
That would keep the lights on, heating, water, public transport systems working. Expensive equipment can be protected from new networks.
The main use for this kind of networking would be during a color revolution https://en.wikipedia.org/wiki/....
A government fails to deliver basic services and then all services are quickly restored by a new regime.
The equipment manufacturers are caught between offering decades and generations of quality service as a brand and having their own networking products used by different clandestine services. Not the best marketing or optics for a next bid or new infrastructure project.

Best just to log it all and then sort for any reason later.
The other aspect to massive domestic log creation is to help track undercover staff, staff, informants, whistleblowers, the press and wider legal system.
That haystack can work in both directions. Fully understanding the US telco network and all US social media can ensure a good background story for undercover work and help track all interest in that created persona or court case.
Is a law firm or member of the press too interested in methods that where hidden from a court? Who are they communicating with? What private sector social media databases have they searched? What did they find?
Did facial recognition find a photo on old or new social media that any private investigator or member of the press could find given the vast private sector collections of social media?

Re But with strong crypto it's secure enough that the 3rd party can see (or alter) your communications.
With international crypto standards set by tame groups that failed in the past and offered junk crypto?
All a nation has to do is work out who is connecting and with what software products. Encryption can be removed by ensuring the wide use of junk encryption.
Low costs, free, tame academics, NGO's, foundations, front companies can all push a message of testing, security and options around tame crypto.
Any good quality encryption is lost to the flood of well funded weak products offered for free or the need to use a tame international standard.
The producers of good quality encryption can also be found, their product can be weakened or sold to a more tame brand.
If good encryption exists for free then the users are tracked and tame operating systems or hardware might allow the plain text to be recovered.

There are two ideas on that topic. The past idea was the vital importance of keeping all public spying topics limited to the Soviet Union, Russia and China. That kept the domestic press happy and the simple domestic message that the internet was too big and fast to "collect it all".
The other idea was to make all collection legal and use the results in secure or open courts.
Tempora https://en.wikipedia.org/wiki/... shows that it is possible to reconcile a vast usage database removing all anonymity and privacy.
The security services worked out decades ago that telling people that crypto was broken or that all communications was been logged changes how people use networks and telecommunications services.
Sock puppets and propaganda could still contain whistleblowers and their material in the press.
Now nations are talking of decryption and collecting it all openly. The past 90 years of well hidden surveillance is now out in public and legal.
A digital Berlin Wall is now legal and public.

Wouldn't it make much more sense to devote some effort and expense to promoting the values of the enlightenment?
The CIA did that with modern art.
Modern art was CIA 'weapon' ( 22 October 1995) http://www.independent.co.uk/n...
"the CIA fostered and promoted American Abstract Expressionist painting around the world for more than 20 years."

Why?
Part of collect it all. Bring it out for use in open courts. Everything collected is now more legal. No more parallel construction needed.
All that new data can then be shared with other EU, UK and US beyond the "The Ring of Five" Germany, the Netherlands, France, Belgium and Denmark.
The part most nations dont seem to have fully understood is what the GCHQ understood decades ago. Never mention the collection part and people keep on trusting their phones and computer networks.
Now that nations admit they have full mastery of all their internal phones and computer networks interesting people can just revert to more traditional methods of communications.

False positives are an equal problem.
Three prisoners in a detention camp get to talking about why they are there.
"I am here because I always sent too much spam, and they charged me with been a numbers station," says the first.
"I am here because I sent direct marketing messages, and they charged me with helping sleeper agents," says the second.
"I am here because I sent an email every day," says the third, "and they charged me with been a sleeper agent."

Re " Like every letter sent" was under consideration from some types of communications.
Project SHAMROCK https://en.wikipedia.org/wiki/...
"started in August 1945 that involved the accumulation of all telegraphic data entering into or exiting from the United States. The Armed Forces Security Agency (AFSA) and its successor NSA were given direct access to daily microfilm copies of all incoming, outgoing, and transiting telegrams via the Western Union and its associates RCA and ITT."
Just the early days of collect it all.
The UK had Defence of the Realm Act 1914 (DORA) https://en.wikipedia.org/wiki/... to help with letters.

A VPS could be given an offer by security services in its host country. Become a honey pot for all foreign networking traffic that expects privacy and anonymity.
With tame international standards and all ip's been logged a person is left with the security and privacy of an application. As both ends of the encrypted chat are uncovered the only task is to get the plain text, voice before encryption by an app. Most open and consumer grade OS seem to be very useful to offer access to plain text as entered or a voice stream before encryption is used.

Consider the interesting parts of the internet to be Tempora ready https://en.wikipedia.org/wiki/... .
Every message in is reconciled with every message out. Finding the end and entry ip is then easy at a county, state or federal level.
People may expect privacy and anonymity from networks that are collecting it all.
If Bob and Sally are under constant constant surveillance privacy would be an issue.
If Bob is under active surveillance and Sally as a journalist is, then anonymity for any whistleblower is interesting.
Or every browser could be requested to leak the real ip. Tails or Whonix could offer more but users have to be aware of privacy and anonymity.
Entering Tor as a totally different person every time is task that has to be understood every session.
With every line of text linguistic analysis hopes for a bit more insight.

The other question is a Tails https://en.wikipedia.org/wiki/... or Whonix (Tor anonymity network, Debian GNU/Linux and security by isolation) https://www.whonix.org/
That would in theory contain any more direct ip requests sent from any site or network.
Re "how would anyone with a basic understanding of networking not see?"
funded by the US government (16, 2014)
http://pando.com/2014/07/16/to...
The parallel construction that still seems to hold up is the sending of a page or code to show the real ip that always seems to leak out.

It was regrettable security researchers, brands, firms, academics and other experts failed to find, did not look, did not ask, did not consider, did not want to understand, where not interested or collaborated in placing so many trap doors and backdoors in international crypto standards over the years.
Just getting weak crypto created and set as a standard is the first part. Keeping it as a standard for some time was the real trick. At lot of smart people and top brands had to stay tame and look the other way on that aspect over the years.
The good news is people can just move back to number stations and only use one time pads once.
The intentionally create algorithms seemed to go back to the 1950's as the Martin and Mitchell defection hinted in the early 1960's
https://en.wikipedia.org/wiki/...
"Our main dissatisfaction concerned some of the practices the United States uses in gathering intelligence information ... deliberately violating the airspace of other nations ... intercepting and deciphering the secret communications of its own allies ..."