This is an excellent question.
But, if you look at the history of CFS, you will realize that this diagnosis was "made up" for patients with myasthenia-like symptoms and normal tests.

It was a well-intentioned psychiatrist, http://rg.kcl.ac.uk/staffprofiles/st...le.php?pid=932 who tried to find the way to help those patients who were referred to him by frustrated neurologists, but (like happens many times) found the way to help himself instead.

In fact his land-mark paper (with numerous methodological problems) comparing MG patients to CFS patients is one of the most cited.