how come despite spending billions of dollars pharmaceutical companies are not able to find the magic bullet to cure mental disorders

CT must be no more than 210 words

(Note: your response must exclusively focus on the biological/anatomical/physiological aspects. Any response that is based on social/economic factors will not be accepted. In the past some students focused on expressing views on the evil intentions of the pharmaceutical companies (such as: these companies deliberately refusing to find the magic bullet; or that these companies already have developed the magic bullet and refuse to market it, because it is not in their economic interest). Those students received score of zero for this CT)