Part of the issue is the power psychiatry wields as a profession, relative to psychology.
“Evidence-based” usually translates to privileging biochemistry over psychology (the latter of which is “soft”, and therefore “less empirical”).
People have equated the brain with mind more and more, over time. Pushing that further, if neural dynamics determine psychological states (such as moods) outright–but, not vice versa–it becomes logical to target biological factors to uproot behavioral problems.
It is telling that psychological diagnoses are outlined in a manual published by the American Psychiatric (not Psychological) Association. Much of modern psychology grew out of psychiatry–see Freud and Jung, but also other theorists around their time like Karen Horney, who was an M.D.–so part of this is attributable to psychology’s heritage.
As the field develops its identity more, perhaps we’ll see psychologists push for greater control over the field’s diagnosis-establishing processes.