IMO, on Mastodon for example, most people don't understand federation and don't care about it. It sounds scary and complicated to them. And also in Mastodon's case at least, it's still extremely difficult IMO, to find people/sites/etc. to follow about topics you care about. You still, AFAIK, can't search by posts which is just dumb, IMO (at least on the instance/server) I'm on.
The whole pick an instance/server thing also confuses people. Most people don't care about that and don't even need to know TBH. Just auto-assign them to the most popular one and those who care about such things can manually pick their own.