-We dont understand human brains enough.
-Psychotherapy is extremely primitive, its like prescribing leeches as treatment. Basically try a bunch of "treatments" see if it improves. (not if it fixes.)
-Besides, it might be completely flawed to begin with. When you are hungry, you dont go: "Jeez, there is a chemical imbalance in my brain, i should take something so i dont FEEL hungry anymore." No, you just eat something. The feeling is not the cause. Being hungry didnt make you mentally ill.