Probably because it’s fast, scales well in terms of size, and decently easy to build apps around. Shouldn’t the question be, why not?
Well, if you're self hosting for yourself, friends, and family.. it isn't likely to be a thing you'll want to care about fixing when it eventually breaks in mysterious ways.
Better to use sqlite or just a blob of yaml in a file if self hosting might be involved.
Would have preferred a more unsurprising db like psql or SQLite for my usage, but they also support data dumps so if needed I can escape.