If you are rich you have single-use rooms. Hell, even I have a sleeping room, a cat room and a hacking room. It's not that expensive. The cat room doubles as a mini-cinema.

Ambient light is dealt with having nice thick blackout drapes.

This is not some rocket science, it was done for like a hundred years.

edit: actually in my former apartment I just bought a roll-down projector screen that completely covered the window. pull down - zero ambient light even in full sunshine.

Lots of people (even exceptionally wealthy ones) don't want to always have to go to the theater room to go watch something. Lots of people like having the TV on in the same way one might think about having a radio on, its not necessarily a thing they're exclusively focused on.

Take a look at this house:

https://www.zillow.com/homedetails/5619-Walnut-Hill-Ln-Dalla...?

Sure, it has a theater room with probably an incredible projector I'd imagine. It also has a bar with a big TV behind it. It also has a game room with several TVs mounted on the walls. In both of those situations, you wouldn't really want a projection setup. A brighter TV wins out in the end.

I've seen lots of TVs mounted in outdoor living room/kitchen arrangements. They're common to find in living rooms even for people who have separate theater rooms. I've seen them mounted in garages. TVs have uses other than just pure cinematic experiences. And I say that as someone who has personally pushed TV watching to an exclusive space in my home and made a point to not have TVs everywhere. Other people like different things.