"Humans act" is a synthetic axiom, that is, it's really a pseudonym for a group of axioms, each of which can be understood (though not proven) as following from the fact that humans act.

Also, bear in mind that when we say "only axiom" we implicitly mean the "only economic axiom" since the axioms of logic and so on must be supposed in order to even have a discussion. The law of identity does not follow from "humans act."