I think that most mental illnesses will almost certainly turn out to be primarily caused by neurological and immunological abnormalities, rather than psychosocial or psychological factors.
I think the reason that historically — and still to a large degree at present — psychiatrists have looked for psychosocial or psychological factors, rather than neurological and immunological factors (physical factors), to explain mental illness and mental symptoms is because:
(1) Until recent decades, we did not have many ways of peering into the brain or immune system to search for dysfunction. So this caused a focus on the psychosocial or psychological factors which were accessible to study, but not the neurological and immunological factors which were not accessible. But technology is changing this.
(2) Many psychiatrists and psychologists do not have the mindset and skill set to examine underlying neurological and immunological pathophysiologies of mental illness. Psychiatrists and psychologists probably have a good amount of empathetic skills, which enable them to enter into the minds of others; but they may not necessarily have the ability and skill set to navigate through neurology and immunology. Thus by their own inclinations, they may avoid considering the brain and physical side, and focus only on the mental side of the illnesses they study. Which is I think the wrong place to look, if the causes of mental illness are primarily physical.
As technology provides us with increasing observational access to the brain, immune system and other physical aspects of body, there needs to be a new breed of psychiatrists and psychologists that can academically master of both the mind and the brain, and can then attempt to figure out how abnormalities in the former are caused by dysfunction in the latter.