There are bootstraps you haven’t appropriately pulled up if you live at home.

The more legitimate reason is that there’s a school of thought that you can’t become a fully-fledged, independent adult without putting some distance between youself and the folks that raised you. There’s a difference between someone who never left home and is content to just stay in the status quo, vs an adult who maybe went to off to college or was away from home for some period of time while working that has had to come back due to challenging circumstances and doesn’t plan to stay longer than they need to. Obviously, the stereotype is of the former and not the latter.