It's amazing how far the medical world has come in treating ailments of the body, but we've made comparatively little headway in ailments of the brain.
If instead of saying brain you say "The most complicated structure found in the known universe so far" it starts to make a little more sense on the enormity of the problem at hand.
Most organs are composed of one or only a few types of cells and their particular arrangement doesn't matter that much. For example swapping kidneys between people doesn't change who they are (mostly, smaller edge conditions apply here). But we can't swap brains because the brain encodes your 'youness'.
While true, the complexity of each organelle in the brain is still much higher than that of the typical organ elsewhere in the body. That and there is a high degree of connectivity and feedback between components where the API is completely and totally undocumented. It's not that you don't know what the known API calls do, you don't have any clue on how many API calls there even are. Accidently emulating one of these calls when attempting to trigger a different behavior can lead to wildly unexpected outcomes.
You’re correct overall, and I know what you mean. But to offer a small and somewhat provocative nitpick: these (the psychoses or any other so-called mental illness) aren’t “ailments of the brain”, as they’ve never been shown to be. None has any demonstrated underlying pathology, much less a clinically useful one.
Mechanical damage to parts of the brain can cause personality changes. Multiple neurological syndromes are directly linked to macroscopic brain changes. This includes schizophrenia, traumatic brain injuries dramatically increase its risk.
We also know that schizophrenia almost never happens along with epilepsy. They are almost mutually exclusive.
It's fair to say there is no single macroscopic trigger for schizophrenia, but that's it.
Schizophrenia is directly linked to dopamine. So is ADHD, but in the opposite way (not enough dopamine). Both were demonstrated on fMRI.
Psychosis is a physical issue in the brain (too much dopamine making the brain fire when it shouldn't) that's solved by administering dopamine antagonists (= antipsychotics).
There is little surprise that if a brain is behaving differently, that we can see differences in neurotransmitters. Like the Serotonin Hypothesis, that doesn't mean the neurotransmitters themselves are part of the physical cause and not just a consequence.
It's like trying to diagnose and treat a political crisis by observing telephone connections. We see stark changes in telephone use in a crisis. If we meddle with the telephone exchanges we might improve or harm various types of event. It's not really about the telephone calls though, we are just hacking at the messaging of an external event we are ignorant of.
A conspiracy theory induced panic might be superficially "solved" but cutting all the the phone lines. We have not identified or solved the cause though and people don't find life without telephones (dopamine) worth living so they plug them back in (quit their anti-psychotics).
There are dozens, but this is one of the most clear and to the point, and is found in one of the most mainstream, orthodox, and hard-to-get-published-by journals in mental health:
Weinberger DR, Radulescu E. Finding the elusive psychiatric “lesion” with 21st-century neuroanatomy: a note of caution. Am J Psychiatry. 2016;173(1):27-33.
The situation is about as bad as it can be: there are so many potential sources of confounding in MRI studies that they simply cannot be trusted to say anything about the basic cellular elements of brain tissue in either healthy or diseased patients. All they can validly talk about is themselves; in other words "rather than referring to differences as evidence of brain structural abnormalities, they should be called differences on MRI measurements."
This is without getting into other hugely problematic aspects of MRI studies in mental health: tiny samples; failure to replicate; and most interesting to me, the fact that experimental subjects (i.e. the people in the study "with ADHD" or schizophrenia or whatever) are not screened for psychiatric medication use, which, because medications can shrink brain tissue, can make it look like the condition is affecting brain structure when in fact the drug is. (So another source of confounding, then.)
Last I checked my brain was part of my body. And that is why so little headway has been made, people keep thinking the body and brain act independently of each other.
Most organs are composed of one or only a few types of cells and their particular arrangement doesn't matter that much. For example swapping kidneys between people doesn't change who they are (mostly, smaller edge conditions apply here). But we can't swap brains because the brain encodes your 'youness'.