There are plenty of news subreddits. I greatly preferred /r/anime_titties, for example.
On Reddit, since there's only one "instance" and can never be any others, there can only be a single community named "news." If the name is really so important then the ability to "reuse" the name on other instances gives an advantage to the Fediverse.
People don't want to post in two communities that cover the same topic.
Then don't. I really don't understand what you think is going on here. If there's one community you prefer, stick with that.
If you fragment users into multiple instances
They are not fragmented. In what way are they fragmented? Everyone can participate in communities on every instance, no matter where they are.
people won't leave that instance
People don't need to leave that instance.
In other words: why go to !worldnews when there's so many more users, and so much more fresh content on https://reddit.com/r/worldnews?
Because Reddit's admins suck? Why else are you here?