As an insurance claim adjuster for 30 years, I first began to see this diagnosis about 1995. Keep in mind, "fibro" means muscle fibers are involved, and "myalgia" just means muscle pain. What I observed is that there is no evidence for this diagnosis. Many illnesses and diseases can be verified with a test. Many physical ailments can be verified with X-Ray, MRI, CT scan, etc.
With fibromyalgia there is nothing to see, no test to prove it. It seemed to be a term developed for otherwise unexplained pain. It could be real, it's just not objective.
I became skeptical because it became such a popular diagnosis for people involved in auto accidents, usually those who retained an attorney. Again, this is not evidence, but I'm just explaining the basis for my doubts.
So, whereas I had been settling claims for a few thousand dollars when people were stiff and sore for several weeks, with fibromyalgia the argument became that the accident didn't just cause temporary muscle sprain or strain. Now it was a debilitating crippling pain with no discernible cure or resolution. And all with no physical or medical proof, but requiring much more money to compensate the victim.
So, this doesn't answer the question, it is just one man's experience.
Or hear me out, maybe those companies were undercompensating people who had real pain before but couldnt prove it and didnt have a name for what was happening to them.
There's a lot we still don't understand about pain. We certainly know certain things that can cause pain, but there's a lot we don't know.