Medicine is supposed to be science based — unenlightened
Medicine is a practice, and in so saying I'm not knocking it. Ditto for psychiatry. Ditto for dentistry. On their way to practice, students study science (like, major in molecular biology), then take more classes, and rotate through clinics. Along the way a good deal of solid science is encountered and (we hope) absorbed. But let us face a fact: The various sciences behind medicine, and all their content, is way too voluminous for the brightest doctor to carry around in his or her head.
Students mostly learn how to be doctors and dentists (and psychiatrists) by
practicing on patients. Once they get good at it, they keep 'practicing'. They go with what works, what makes patients happier, or at least not dead.
@Hanover, lawyers used to prepare for the bar just by reading and practicing. Abe Lincoln became a lawyer that way. Worked for him. The Mayo brothers weren't master scientists, they were very good organizers.
So, a psychiatrist is presented with two different patients, The first is clearly out of his mind -- screaming incoherently, flailing about, totally nuts. The other patient is unhappy, is doing poorly in life, but is functional. What to do?
In the first case, administer Thorazine, put him in a padded cell, and wait for the drugs to work. Then out of the cell into a locked ward, then into an unlocked ward, and eventually, home with an Rx for lithium. The psychiatrist doesn't need to know (and doesn't, in fact know) how Thorazine and lithium work, just that they do what they do. Patient gets better.
The unhappy patient doing poorly in life, but who is behaving more or less 'appropriately' presents a lot more difficulty in a way, because there is no particular drug or intervention that will dramatically change his behavior. From his practical experience, the psychiatrist knows that soothing words help, some drug or placebo will help; encouragement, helping the patient develop some insight, and so on may all be helpful--or not.
Where psychiatrists really earn their status is in dealing with major mental illness, where life and death issues are at hand. Their waiting room full of merely unhappy, dissatisfied, pissed off, worried sick patients will mostly get better on their own, as they always have, but he gets paid to help them, so...