Because now you're on the hook to maintain that fork forever. And because your fork, by definition, has diverged from "the mainline" because there really isn't any "mainline" anymore. You might have 2 or 3 or 10 or 100 other people who also forked just to fix one random bug or whatever, but there's no real cohesion or interaction between all these isolated forks.

Best case, one of the forks clearly emerges as "the" new primary repo and everybody shifts their focus there. But that doesn't always happen. It's a real problem with the fork-centric model that Github promotes. Of course, one could argue that this state of affairs is still better than the "old way" where forking a project was a very heavyweight process and rarely happened at all. But that doesn't mean the current model couldn't be improved further.

Is this Citizenship idea the right way? I don't know, but - at worst - it's a good discussion to have.