Olanzapine, it's not reasonable to malign the entire profession of psychiatry based upon your individual experience. In any given field, of course there will be more competent and less competent people.
Becoming a psychiatrist requires a four-year residency, by the way, with training programs offering various emphases on different therapeutic modalities (including pharmacotherapies and psychotherapies,) psychobiology, neuroscience, and research.
I'm not sure whether psychologists should have prescriptive privileges or not, but I do think that any good psychiatrist should have a solid foundation in medicine first. This is essential in order to be able to discriminate between psychiatric illness and psychiatric manifestations of other systemic illness, to understand the best side effect profiles for people with various co-morbidities, and to prescribe medications appropriately given potential interactions with a patient's other meds.
I imagine that if a psychologist were to undertake all of a patient's psychiatric care (perhaps via assuming prescriptive privileges?), there would be the risk of overlooking something significant, simply because the provider did not have a medical background. Furthermore, as psychiatry (or at least some part of the discipline) moves towards an increasingly biologically-based orientation, it doesn't seem practical or efficient to expect psychologists to jump aboard and develop expertise in neuroanatomy, neuroimaging, etc (nor do I imagine that many would want to.)
In mental health, as in so many fields, the work seems to function most smoothly with a multi-disciplinary approach. There are plenty of opportunities for psychologists, psychiatrists, social workers, nurses, etc., to all make important and valuable contributions.