Because we're "crazy" or "out of focus." Better for the psychiatrists to trust the drug companies that give them company written books on what is recommended for people with psychological problems, despite their huge incomes due to "medications" that only show that "something" happens; there's no proof that the level of serotonin, for instance (controlled with so called "SSRI's" (a company created name, by the way)), does any improvement in the mood. Much does, however, show that people often get more depressed, suicidal, or may, at times, even kill people. Despite this, the companies have managed to convince the whole world that faulty serotonin levels are a cause of depression. What it really is is just a massive social experiment.
I do, however, think that a select few may in fact be helped by some of these substances, but they are few and far between... the vast majority is just unknowingly participating in an experiment based on greed. Or, actually, all, including those who happen to get better, are participants, but the companies must of course manage to get a select few better, just by luck.