That’s because it isn’t.

(No, I won’t go into a discussion how language changes and things that used to be mistakes are no fine and yaddayadda. In today’s standard English you cannot contract ‘I have’ if ‘have’ is not an auxiliary verb.)

Note that the have verb is not contracted in writing when it is the main verb in a sentence and means “to possess.”

https://editorsmanual.com/articles/types-of-contractions/