Afaik, English grammar requires utterances with predicates to have a stressed element in those predicates. Contractions of only a subject and an auxiliary verb - ex: I am > I'm, he has > he's, they will > they'll - eliminate that independent auxiliary as a prosodic segment and violate that grammar.

A - "Who's going to the store?"

B - "I am." [ok] or "I'm going." [ok] (or "I am going."), but not "I'm." [bad, obvs].