Really the issue is that psychology is in its infancy. Diseases that are well understood are defined and diagnosed by their root cause - the presence of a certain pathogen, a dysfunction of a certain organ. But when medicine doesn't understand the cause of a disease, psychological or otherwise, all that can be diagnosed is a cluster of correlated symptoms.

Take, for example, Irritable Bowel Syndrome - no one exactly knows what causes it, or if it's even a single disease, but there are a cluster of symptoms that indicate that a certain set of treatments might be useful (or not, depending on the patient). It is likely also a cluster of similar-looking diseases. Sounds a lot like the state of psychology, yeah?

Hopefully as we start understanding the brain, we can start diagnosing actual diseases with known causes, and not just a set of associated visible issues.