That might be like asking someone with diabetes "Why don't you break down sugar in your body?"

The answer is "because it is an illness". Something is wrong. It is a question that can't help make you feel defensive and helpless posed but someone ignorant that mental illness is an illness.

The question implies judgement as if there should be an easy answer but there is no easy answer that's why this is such a difficult illness to deal with. The fact that you don't have an answer shouldn't make you feel worse the person who poses such a judgemental question is the one who should feel bad.

Wish we lived in a world where people would at least try to understand that if they don't know something it means the answer may be complex. Instead many assume that if they don't know something the answer must be simple.