I've wondered about why they're called "personality disorders" also but one of the possible reasons I can think of is they've been with you for such a long time and are integrated into you, much like one's personality is integrated. Also, the treatment can involve medications but would also focus on changing how one views things, interacts, etc..., which is similar if not the same as one's personality. As for why it's considered a disorder, I can find multiple reasons. One of them could be society's view on it in that it's inappropriate or violates norms. Another possible reason would be that it's a statistical inconsistency in terms of although many people may have that condition/disorder, it's still an infrequent disorder. Because it's infrequent, and it has maladaptive symptoms or whatnot, put two and two together and we get a "personality disorder". That's my summarized view but there probably is more to the story than just this.