I asked myself the same thing... I guess mostly because no one has ever suggested it. I've been in therapy most of my life. But I guess when I was a kid, autism wasn't really known. And later in life, I guess it's possible that no one looked for another explanation when I already had diagnoses.
I don't know. But does it really matter?
Lately I'm thinking maybe I'm focusing too much on symptoms and categories and naming what's wrong with me. I thought I needed to know, to know how to fix it. But maybe it doesn't matter as much as I thought. I know the effects of whatever it is. I know the problems it causes, I suppose any kind of therapy I can be in will focus on the problems, not the diagnosis, right? Or at least they should.