I also often pair with infrastructure people on solving a problem - e.g. "im trying to do x as per the docs, but if you look at my screen i get an 1003 error code any idea what went wrong?".
Or, people on a different team whose microservice talks to mine when debugging an issue or fleshing out an API spec.
It's true that this isnt possible in plenty of organizations due to the culture, but lots of organizations are broken in all sorts of ways that set piles of cash on fire. This one isnt unique.
I haven't tried pair programming except in very ad-hoc situations, but doing it all the time sounds utterly exhausting. You're taking programming, then layering on top of it a level of constant social interaction over it, and removing the autonomy to just zone out a bit when you need to (to manage stress).
Basically, it sounds like turning programming into an all-day meeting.
So I think it's probably unpopular because most software engineers don't have the personalty to enjoy or even tolerate that environment.
[edit] I’m not even anti-social, but the feeling of being watched while working is extremely draining. An hour of that is like four hours without it.
But it's something you have to work at which is definitely part of the barrier. Otherwise, saying it sucks without giving it a real try is akin to saying, "I went for a run and didn't lose any weight so I feel that running is exhausting with no benefit."
I find it depressing and dystopian that people are now excited about having a robot pair.