The ml posters are aware of other instances.

Yes, but most of them just want to stay on the most active community and don't want to go through the hassle of recreating the community elsewhere. Discussion on !fantasy@lemmy.ml: https://lemmy.ml/post/25858077

Everyone is free to use a instance that federates how they like. If people choose an exclusionary instance then they can’t complain they are missing posts from something excluded.

Correct, but people are also free to want their own version of the community to thrive. Which means posting content to it.

I absofuckinglutely mind seeing redundant content in my feed. It makes Lemmy feel empty

Aren't most of the crossposts only displayed once?

For the remaining ones (usually self posts, as there is no link to identify crossposts), just block the communities that you know are only "other communities reposts"