I've thought the same thing. When someone has the flu, the doctors don't say "push on and ignore it," or "keep on working but pace yourself." They say "rest in bed, drink plenty of fluids, and take something for the pain."
What's more, when you go to the doctor and say "Doc, I feel awful. I ache all over and I feel sick. I think I've got the flu," they don't say "Prove it." To me, this feels like a flu that never goes away. Why do I think there's probably a virus or other pathogen causing it? Because that's what it feels like. How do I feel? I feel SICK, just like other people who feel sick. Is that so hard to grasp?
Well, whether the docs get it or not, this disease has taught me what it will and won't allow. When I try to push through it, it kicks me in the butt. The docs can call that "post exertional malaise"; I call it being sick.