How would you get evaluated for a trait independently of a diagnosis? That's not really a thing.

The problem with psychological diagnoses is they are mostly bundles of symptoms that are grouped together into a diagnosis, and it can't be a diagnosis that effects too many people because then it wouldn't be a disorder. That is to say, there is a very serious political component to psychology.

A medical diagnosis can be very targetted to one thing (like celiac disease) and it can be demonstrated objectively.