My Degrees in Psychology, so I feel like I can at least shed a little insight onto things.
Firstly, Psychology is a difficult field of science. You don't have to be concerned that your treating a helium atom in a morally acceptable manner, nor do you have to find a way to get a random sample of Atoms while still filtering out those atoms that won't work in your study. You can't jab a needle into a persons brain and say, "Yep, 20 milligrams of depression". And while other sciences were busy becoming well established, Psychology was getting over Freud ranting about screwing your mother, penises and cocaine. To top it all off, you need to get very precise, insightful observations from people who you wouldn't trust with your order at Taco Bell. Psychology, by its very nature, is about putting numbers and data into things where numbers and data are not necessarily apparent, and requires interpretation and utmost professionalism from flawed human beings. It requires the finesse of an artist and the exactitude of a scientist, a rare combination that leads to a ton of BS, and the public at large is not qualified to sift through the BS to find the important, salient gems. Mental illnesses are real, Psychologists are just busy trying to make the best possible construct for how to interpret the various behaviors. By the time Psychology filters out of the papers and into real treatment, it gets filled with the BS you hear about so often.
Another problem is that there's shades of Grey in Mental illness, and people *hate* anything that's not black and white. You don't always have a person who is dyslexic on one hand, and a person whose not on the other. You have a person whose a little dyslexic, someone whose a little more, a little less, a person whose a bit more intelligent but a bit more dyslexic, a person whose a bit more motivated, a person whose a bit more self conscious about there dyslexia and screws up tests because of it, a person whose dyslexic and a little of something else... there's a million variables involved, and its difficult to take a million variables and slap a single label on it. So people take a complex set of variables, simplify it to x, and say y cures x. Give y, problem solved. When that doesn't work, you blame the doctor. Give the study of Mental Illnesses a break. Your dealing with the single most complicated chunk of molecules in the universe, the answers to problems with it is not likely to be simple.