From a patient perspective, it's pretty believable. They don't know what's wrong after the typical battery of tests, so it's incurable or you're making it up.

I've also had an experience where I basically told the doctor

>I am tired all the time, I sleep 12-16 hours a day and still wake up feeling completely unrested, and it seriously affects my ability to do my school work and live my life

and his response was (to paraphrase)

>you just need exercise, fatty

For context, I was in my late teens at this point, and a few pounds overweight by the medical definition.

Long story short, after many expensive visits with psychiatrists, we learn that anti-depressants didn't help because I wasn't depressed.

I finally did a sleep study, and it was sleep apnea. A sleep disorder.

This sounds ridiculous, right? As an outsider looking in, it's so obvious. But this actually happened to me, and I am sure it happens to thousands of patients all of the time.