I expect a lot of companies just don't really know how things are going to play out right now. So, unless you know that a position can be 100% remote for all time, the conservative thing to do is to basically say "You're going to have to live somewhere that allows you to commute in a couple of days a week." You don't want to put yourself in a position where you've told someone they can live anywhere in the country they want and then, in nine months, tell them "Just kidding. You need to move to London."
Notably, the market is such that a skilled candidate going in and explicitly negotiating how remote they want to be has a good shot at getting it approved. At least if aiming for e.g. 80% remote.
It's also notable that now being someone with longer experience in working remote - especially managing remote teams - has suddenly become a very valuable skill.
I bet a lot aren't being open about how remote they will be also, to avoid putting people off.