I too am arguing that 'psychological' illnesses probably have physical causes.
Well then we are all in agreement.
As a general point: people may not have it clear in their minds the difference in the concepts of mental/psychological
symptoms, and mental/psychological
causes.
In fact, there are four such concepts:
mental (psychological) symptoms
mental (psychological) causes
physical symptoms
physical causes
Any given disease may involve physical symptoms and/or mental symptoms. But the causes of those physical and/or mental symptoms is a distinct issue, and these causes may be:
Purely physical causes (even for purely mental diseases)
Purely mental causes
A mixture of physical and mental causes
My view is that too much of psychology assumes that mental symptoms most likely must have mental causes (a sort of "like causes like" philosophy). And that too few psychologists work from the basis that mental symptoms may well be due to physical causes.
Sure, in mental disorders, there can be dual causal factors: as per the GAD example I gave above: if you develop GAD from a physical cause, you then become extra sensitive to stress, tension and uncertainty in the world around you. Thus if your world is full of stress and uncertainty, compounded with your physically-caused GAD, this may make the symptoms of your GAD condition much worse.
Similarly, ME/CFS patients have a disease that most likely has a physical cause, but this disease make you extra sensitive to stress and social discord, such that when you are exposed to these (they are to be considered mental causes), your ME/CFS symptoms may actually get worse (in a PEM-type response).
So while there is no denying that mental causes (such as a stressful environment) may exacerbate a mental disorder, the primary etiology of many mental disorders is most likely to be underpinned by a physical cause, in my opinion.