Hell, trolls could go around and recreate accounts on the top 100 instances with the same username users have on other instances to prevent them from reusing the same username elsewhere, just that is a weird concept to explain

Yes but that doesn't mean you should get automatic dibs on a name everywhere. It's just a name. If you are Joe Bill at lemm.ee, that does not give you any rights over the name Joe Bill all across the world. Statistically speaking, there's at least 18 thousand other Joe Bills around at this very moment.

Like, this is something that is already solved by the instance's moderators.