This isn’t necessarily true. There’s a wide variety of reasons for which diagnoses of mental illnesses would vary greatly. Equating Psychology to hard sciences is naive. It is the softest of sciences, but labeling it as non-science would be ridiculous.
I personally think Psychiatrists in particular have one of the most difficult jobs as physicians. It’s made worse for patients by the fact that a poorly performing Psychiatrist has many scapegoats behind which to hide.
Unfortunately I spend a lot of time around mentally ill folk, myself included. A good dose/choice of medication and a good therapist-client-match work modern miracles on people, and it’s very easy to see when you’re around it. It’s very similar to a surgery with a low success rate.