The reason is quite simple, from a pragmatic point of view; amn't is just not used nowadays (many many years ago [>100] it was, in some dialects abbreviated to an't or ain't). Then aren't was used to replace it. Informally, and especially in Am E, ain't is used. Formally am I not? is used - but this is very formal and can sound stuffy; it often has a pompous ring: A teacher who says "I'm right, am I not?" doesn't expect to be contradicted.
Student or Learner