It’s because companies like Meta want all the power they can get. As you said, there’s no reason for them to join the fediverse, other than to control it or kill it off, that is.
I’m not against Threads existing, especially with the way Twitter is going. People need an alternative and I don’t believe that Mastodon is the answer for many.
But Threads and the fediverse can absolutely exist separately, and is why I support defederation.
I’m really worried about the state of GTA 6 if and when it ever comes out. It’s all about money and not the games for Rockstar now and it’s been that way for some time.