I can see how it is a relief, but that does not make it credible. Seems depression, anxiety, rage, obsessive-compulsive, etc are not reducible to simple explanations or causes. As someone said, depression is an abstract feeling, it is not a disease, it is a symptom of poor health on some level. Could be emotional, psychological, existential, physiological, situational, spiritual, etc.

As for medical conditions, you'd have to be more specific cuz not sure what you mean. Things like chronic infections, chronic toxicity, severe nutrient deficiencies can cause symptoms of "mental illness". But neither therapists nor psychiatrists are in a position to treat such things. And when therapists use the word "treatment" seems they are stretching that concept quite far.

I think the MH system gets it wrong so often because it is incapable of comprehending the complexity of human beings and the human body, preferring instead reductionist concepts that avoid the mess and the confusion, and that sell a lot of drugs and a lot of therapy. I personally reject the idea that depression or anxiety is a disorder that we cannot manage or resolve without "treatment". So too the idea that these problems are not simply a natural response to our life situation.