I agree with your claim, but in my experience I encounter people who explain mood disorders as chemical imbalances mostly in a different context. Where I see it

I agree with your claim, but in my experience I encounter people who explain mood disorders as chemical imbalances mostly in a different context. Where I see it most often are good-intentioned people trying to argue against stigma and poor treatment of mental illness. The idea is that there is indeed a physical aspect to it and not just "all in your head", and hence should be treated with the same sympathy toward the patients as physical illnesses, and this is directed toward people holding a certain mindset that stigmatizes mental illness as an argument to not do so. Of course, this is a terribly insufficient characterization of mood disorders. Perhaps this characterization in a game of telephone eventually became misconstrued in the popular consciousness as essential to the disorders.