But the subject and the object are different entities, logically if not physically speaking. The "it" is an unknown, the object of the inevitable preceding question, "Who is it?" The purpose of the answer, "It's me," is to identify the unknown "it" (subject) with the known "me" (object). But until they're identified, they remain different. The reason we don't say "It's myself" is because we aren't told who "it" is until the act of speaking the sentence is complete, so it wouldn't make sense to say "myself" which refers back to a known subject, which in this case we don't have. Whereas "I shaved myself" begins with "I", so the subject is already identified with the object before the object shows up in the sentence.

I think we could resolve this dilemma with a coherent answer to the question "Who is it?" that doesn't involve the verb "to be."