At least here in the US, psychiatry is a specialization of medicine, and a psychiatrist is an MD, while psychology isn't a specialization of anything, and its practitioners' credentials vary widely. The disciplines don't really have much inherent overlap; nobody goes to a psychologist for a prescription, which they can't provide anyway, and nobody really goes to a psychiatrist for "talk therapy", which they mostly prefer to leave to the psychologists whose primary realm it is.