@Facet Because it morphed into ain't.

No, really, this is the etymology of ain't.
-F

@Felthry @Facet was just going to drop in to suggest that

@Mycroft @Felthry Which begs the question, if ain't is considered slangy or not classy/formal, why isn't amn't also still in use?

@Facet @Mycroft I don't think amn't was ever in use; it was a construction that originated in AAVE and then soon morphed into ain't. Pretty sure it was considered informal from the beginning due to its origin, Because Racism™
-F

@Facet @Mycroft In other English dialects, the construction "I'm not" feels more natural than "I amn't", likely because of the two consecutive stressed vowels in the latter? I'm not sure, the reasoning behind that is probably pretty interesting though. But for whatever reason that restriction wasn't a problem in the AAVE dialects that "ain't" originated in. But anyway since the subject of "am" is always "I", there's never any other case to use it, so it always becomes "I'm".
-F