It's a cultural issue, not a technical one. When Mastodon started to take off, one of differentiating factors was about the sort of people they wanted to attract. It's very easy to market to a tribe and appeal to their identity, so it started to stick.

One of my goals with communick is to get rid of this idea, actually. I try to make the point that an instance does not say anything about who you are or who you should follow.