I think doctors just have the weight of experience of those who have tried to "help" those with mental illnesses, whatever experiments or studies have been done. Since a doctor's experiences has to be what he "sees" and works with, it's going to be a bit like a male obstetrician telling a woman how to have a baby :-) But, with the baby, that's almost wholly physical (like a broken leg) so they have an easier time of it than with an individual's mental health, since no two individuals are quite the same in their mental health and there's not much they can "see" other than some observations of behavior perhaps or what they relate as their experience.

I don't know that individuals know much about mental/emotional problems other than their own, whereas the breadth of a doctor''s/therapist's experience trying to help many different individuals with different issues might make them a bit more help than we on the "inside".