You're right. Unfortunately, it's the conditions that lie in the realm of conjecture and opinion that are the province of psychiatry. Sometimes the conjecture is good, sometimes it's not.

When the actual biological cause for a condition is found, it moves over to being treated by other medical specialties and loses its psychiatric significance. Parkinson's, now treated by neurologists is a one such example. Hepatic encephalopathy from liver failure, which can cause dramatic personality changes, is another.

Sometimes its changing social conditions that help recategorize a diagnosis. Case in point, the vote to remove homosexuality as a psychiatric diagnosis from the DSM in 1973.

Psychiatry is the catch-all until we actually figure out how to fix something. You can extrapolate this: if something is a psychiatric diagnosis, theories as to cause and treatment are actually hypotheses, sometimes uncomfortably out of date. If the treatment works for you, that's excellent. But if it doesn't, realize you're dealing with a bug in the most complex object in the universe. Hitting it with a hammer might not work.