Nothing is stopping them from doing it, and the only reason it might not be already happening is the possibility that nobody has cared enough to bother yet.

And by design nothing much can be done about it. That's the nature of a decentralized platform - it's explicitly set up to share content, and that's what it does, by default. And there is no central authority that can control access to the fediverse as a whole. And that's pretty much that.

And personally, I don't care. I've never bought into the nonsensical idea that the stuff I post in a public forum is in any meaningful sense my property after I post it.