Medical school is biased against psychiatry because so much of it hyper-focuses on the physiological processes going on and downplays the behavioral aspects such as good bedside manner.
Most of the professors in medschool have no behavioral science training. It's like putting a bunch of engineers together at a party and one psychologist. The engineers will all talk Star Trek, Fortan, and Linux, and telling people to go into their field, not psychology.
Psychiatry is also a field where diagnosis is heavily laden on clinical opinion more heavily than labs. Non-psychiatric medical doctors often forget that several "physiological' pathologies also use clnical diagnosis. The science and technology is not to the point where diagnosis is done by labs. Several physicians frown upon this ignoring several pathologies that are based too on clinical observation such as temporal arteritis. (Don't argue that diagnosis is by biopsy. Biopsies can only confirm this diagnosis. A negative biopsy can still be temporal arteritis). In fact, we could actually diagnose several forms of mental illness in a more gross and "acceptable" manner to our physician colleagues if ethics (e.g. take out a person's brain and slice it up), cost, and availability of PET scans weren't an issue.
Unfortunately, psychiatry IMHO is now in something of a stage of development similar to diabetes before sugar levels could be taken--when diagnosis had to be based on clinical observation because the diagnostic equipment wasn't yet there to measure sugar levels. This is because mental illness most likely deals on a level even smaller than that of the cell, but that of the second messenger systems, RNA interactions, multiple brain circuits interacting in ways not even fathomed, and several other reasons. We're dealing with something far more sophisticated than non-mental illness in terms of the physiological processes going on. I'd liken this to a microchip vs. a the vacuum tube primitive computers.
What we do know leaves a lot to be desired, but enough to make a positive difference in most cases. Antidepressants for example, in studies, do work but don't work well. Antipsychotics for most patients will improve quality of life but it won't be as good as it was before the psychosis started for many. We have few cures, only treatments.
Throw into the mix the stigma of mental illness with even several medical doctors having prejudice against the mentally ill, that mental illness is a weird phenomenon for many, and that many professions within medicine bash each other (surgeons vs. anesthesiologists, ER doctor vs. any inpatient doctor, Peds vs. Ob-Gyn).
As an attending I get pretty much zero disrespect as a psychiatrist from my physician peers with plenty of them begging me to work for them because they have too many patients with psychiatric issues needing help. I currently work in a psychiatric hospital and here the psychiatrists have higher standing than the non-psychiatric doctors. In medical hospitals some physicians bash psychiatrists, but the bashing is pretty much all-around with every profession bashing a different one.
20 min ago the father tells me how awful doctors are, and how he wished that his midwife would've taken vacation at some other time. They're not your granola-eating types either, they seem pretty normal.
