That's not being particularly generous. If there is no evidence of a physical problem, you get kind of stuck. Doctors have tried hard to find a physical cause for a number of these illness.

As an example, the CDC spent years investigating Delusional Parasitosis, despite evidence it was mental illness, precisely because the sufferers were so insistent they were truly sick (See bottom of https://en.wikipedia.org/wiki/Delusional_parasitosis). They didn't find anything, and the treatment remains antipsychotics, which do appear to work.