Why are mental illnesses considered as brain diseases?

Mental illness. Historically mental diseases were thought to be manifestations of psychological imbalance. Over the last 30+ years it has become clear that the underlying cause of most mental disorders is related to chemical imbalances in the brain.
Brain is in the body. It shows that we have learned that mental illnesses are medical diseases just like diabetes or high blood pressure for example. For mental illness, part of the body that is diseased is the brain. Also, we can give medicines which affect brain chemistry and improve the illness, so the brain must be involved.
MI. Psychiatric symptoms are a direct manifestation of brain or mind dysfunction.
Mental illness. We believe many of the psychiatric disturbances are correlated to the disruptions in the frontal parts of the cerebrum. This may be the answer.
Education. The more we learn about our diagnostic tools, the more often we find both metabolic and anatomic changes with patients who suffer classical psychiatric disorders. These do affect brain chemistry, and function, and drugs that help mental illnesses do influence brain function, so maybe psychiatry and neurology are coming very close.