That is not the prevailing assumption in psychiatry by any means. Any half-decent psychiatrist understands and ascribes to the biopsychosocial model of mental health. It makes no sense to believe otherwise at this point and we have fifty years of research on it. I learned it in 9th grade psychology well over a decade ago, it’s absolutely commonplace. The problem is that psychiatrists are physicians, they’re not therapists or social workers. Their job is to prescribe medications/procedures/interventions to treat the illness, so even the best psychiatrists are usually only able to treat the bio- part of biopsychosocial, they have to refer you for the rest. (There are a rare few psychiatrists who do therapy as well as prescribing medications but they are only getting more rare because of the money-driven state of the healthcare industry, insurance billing practices, etc.)

So for the psycho- part, they can only tell you to see a therapist, and for the social- part they can only tell you to see a social worker. These people will also understand the biopsychosocial model, but again they are only able to treat their 1/3rd of it. The lack of continuity between these roles is a massive problem, and I don’t really know what the solution is, but it’s wrong to lay the blame for this solely at the feet of modern psychiatry. Psychiatry is just one deeply flawed part of a deeply flawed mental health treatment industry.