What makes psychology exempt from evolution?

Nothing; in fact the grandparent specifically assumes it isn't, since I referred to

the evolution of psychological traits[.]

The point was that I expect psychological traits to have different kinds of evolutionary explanations than "physical" traits. I expect evolutionary-psychological explanations to take place on a different level of abstraction, involving environments with other psychological agents. In particular, I don't expect them to directly involve phenomena that also apply to organisms that don't even have a psychology.

Keep in mind that I'm talking about my expectations going in. I'm not complaining about how the universe works (if it turns out to work in a certain way); this is simply a matter of exposition: the post felt jarring. This could have been alleviated quite simply by an acknowledgement that something counterintuitive was being claimed.