If we had a system where we standardized mailbox sizes to some specification
Done, been that way all my life (I'm almost 40).
and then licensed out some NFC/smart card system to let postal workers/delivery companies open them, then we might be getting somewhere
You mean some sort of key ... Again, done, group boxes have had keys all my life.
but if you could simply buy the relevant thing at Home Depot
Home Depot sales mailboxes, all of which meet all sorts of standard requirements for US Postal Service deliveries.
You do realize that everything you've said has been around for, what, a century?
There are even standards for positioning of the mailbox, not just size.
I'm guessing you're not real observant and haven't noticed that all mailboxes are already the same size, basic shape and location.