I ask the same question again, why put this stuff online at all? Why are critical systems for infrastructure online? Why is anything of any importance for our government and nation available to the general Internet?
Because that's how the information gets from (wherever it is stored) to (the people who need to access it). The Internet is popular for a reason, and that reason is that it helps people get things done quickly and cheaply.
The alternative, of course, is to have the information and the people physically co-located, so that they can access the information only via an isolated network (or by physically sitting at the computer the information is stored on).
However, the benefits of remote access are so great that in many cases it's seen as being worth the risk of allowing it. Whether or not that assessment is correct or not depends on an estimate of how secure the networks are, but also on an estimate of how aggressive, competent, and numerous any hostile intruders will be. Clearly it's possible to get both of those estimates wrong, but I'm not sure that a knee-jerk response of "pull all the Ethernet cables and return to the 1950s" is going to be a practical solution either, as doing so would likely cause as much disruption as an actual attack.
I'm not sure what the solution is, but probably one good practice would be a lot more red-teaming -- i.e. if your network is vulnerable to intrusion, it's much better to learn how a friendly intruder got in (by asking him) and fix the hole than to pick up the pieces after a hostile intruder nuked your network.