To the level of schizophrenia and worse is usually a biochemical problem unless severe abuse was involved. Absence of proper parenting can lead to some functional problems, however, severe mental problems are physical and chemical.
I understand what you are saying, and I agree with it to a point. However, I also believe that mental trauma can manifest itself in chemical changes in the brain. I won’t say that is a definitive cause for schizophrenia; I do believe, however that it can be a factor in some cases.
I deal with disabled people in my line of work. I know that these severe mental problems are physical and chemical. But is it not also possible that the level of trauma needed to push someone over a theoretical edge varies from person to person. One person’s bad experience may well be another person’s mind-altering trauma.