They could, but it would be to the detriment of their core business - engagement. Facebook doesn't want to fragment into million different servers the way that (for instance) Discord does, even though that is how a lot of their users might prefer.

Twitter has it even worse - their whole schtick is a huge chattering town hall. Advertisers and brands want this, users increasingly don't.

I tried to express this in a blog post[0] but the gist is that in their quest for engagement the big social media networks have evolved down a path that is not as attractive to users. The environment (the internet using population) is changing but the big social networks cannot change to meet new needs without ditching the things that make them money.

[0] https://sheep.horse/2022/11/social_media_is_not_dying%2C_yet...

It feels as though the online community is about to circle back in a cyclical way, to semi-private forum-like communities. Atleast that's my wishful thinking. I remember being involved in a local bowmaking (archery) forum almost a decade back. The quality of posts there was something I remember fondly to this day.