Agreed, this would be ideal. Most people don't understand that literally every access point on a given channel is part of the same broadcast domain even if they're totally separate networks. Every AP that yours can see on the same channel slows down the network for all of them, because wifi is half-duplex. Everyone needs to shut up for a second while one host on the channel transmits.

I see more than 20 networks in the building I'm in, so that is already happening.

Most people I know don't have cables going to different rooms, there's a router where the signal comes in and wifi from there. Often there's not even the choice which room or part of the room that is.

Unless your building is relatively new and someone thought about setting up the cables, it can be very expensive to do so later.

> very expensive

To lay a cat6 cable and have it snake over the ceiling in a cable gutter? The cable I grant you isn't dirt cheap but it's by far the most expensive part of that arrangement.