And? Do you know how many different kinds of cancer there are? We're talking about psychogical disorders that affect people's minds and personalities, of course their presentations are going to be very different. That doesn't make them not real or mean it's not useful to group them together. Diagnosis guides treatment, and most depression responds to the same treatments. You're fallaciously assuming that because there's not a single biological marker for named psychiatric conditions that they don't exist. That's not how any psychiatrist sees it.