1. Well, maybe you could do that with your abundance of skill and time, but not everyone running a hospital/bank/other-large-organization has the time or budget to slap together and maintain that kind of amateur hour rope-and-tin cans proxy shit.
2. Not everything people use a computer for happens over a network. Some people write software because they need to communicate directly with hardware devices. You can’t intercept network traffic when there is no network traffic to intercept.
2. I believe we're talking about network accessible devices here. Otherwise why would parent make comment about air-gapping infeasible?
2. The parent mentioned networking, but networking is not the only threat vector that must be accounted for. Malware can spread through removable media, and most organizations outside of the military or federal government simply don't have the sort of security policies in place to prevent that. So the unpatched vulnerabilities people are talking about can still be exploited even without the network stack doing anything.