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You’re absolutely right. But the brain is a biological organ, a biological information processor. Experiences shape it’s physical structure and function. Whether mental illness arises from purely environment causes (eg trauma) or purely nature (eg genetics, physical injury), in both cases the phenomena reside in the physical substrate of the brain.

When someone has psychosis (eg hearing voices), maybe it was caused by a severe trauma or maybe it was mostly genetic. In either case the person is suffering from psychosis and is unable to normally function in society (if severe enough). In both cases, in principle, we should be able to detect the problem at the level of the brain in structure and function, and should be able to intervene at the level of the brain.

The intervention required depends on the nature of the brain change. We can change brain function in many cases by intensive therapy. In some cases sensory inputs alone (via talk therapy) are insufficient to change the brain to rectify an issue. That’s when we have to consider direct intervention on the brain itself either via medications or neuromodulation technology.

I don’t think lay people understand how profoundly mentally ill some people are and that they can really have dramatic improvements with treatment. Lay people think of psychiatrists just peddling drugs to mildly depressed patients or people who are mildly oddly behaved. That’s just not the case. I’m working with people who sometimes literally stop eating because they think their food is poisoned, or attempt to kill themselves with minor frustrations, etc. And many patients, mostly not that sick, come in voluntarily because they feel they can’t handle some problem on their own. You’re right that much I mental illness is caused by psychosocial stressors, but some people have just intractably messed up life situations and they’ve just spiraled downward as a result and wouldn’t be able to get on a better path without treatment.