People disagree with that perspective because it implies that the brain is entirely deterministic biologically and has no plasticity. That you have no mental control over your physiology. That you don't have control over your attitude and outlook, etc.
Attitude is proven to have an effect on cancer remission rates. Yet we still call it a disease and say that it killed you, not that you selfishly killed yourself due to not getting better.
You have some control. Far less than the "suicide is selfish" idiocy assumes.
That fits with my experience. I have to be very careful with peanut oil, for example. It does terrible things to me.
So, in short, it is possible that depression and other mental illness has biological roots. There are also social factors and other factors, but in some sense mental illness may well be a medical illness.
