The symptoms, abnormalities, triggers and cures match perfectly with what you would expect from exhaustion due to chronic stress.
If you didn't know it was caused by a virus, you could say the same thing about having the flu, most autoimmune diseases, etc. ME/CFS causes many consistent and characteristic symptoms that aren't fatigue, and most other chronic diseases cause fatigue. Why should ME/CFS be psychosomatic any more than any other illness for which we don't know the cause?