And why is it that some doctors don't even think fibro really exists?

Because it doesn't. ? many MD's doubt fibromyalgia (FM) real. FM is disorder absent single cause; is "umbrella label" for collection of causes sharing same constellation of non-specific symptoms (chronic widespread pain, restless sleep, chronic fatigue, depression, confusion). Each sufferer w. different causal array (femaleness, childbirth, mechanical injuries, hypermobity, obesity, prolonged illness w. inactivity).
Unanswered questions. There is no known cause for fibromyalgia. There is no good test to diagnose or rule out the condition. The tender point exam was developed for research purposes. The disease is often confused with pure anxiety or depression. Thus, some physicians do not believe in fibromyalgia.