Hi I'm a doctor. Yes fibromyalgia is real. Yes it's probably used as a catchall dx for a group of various mental and musculoskeletal conditions. From what I have seen, it's mostly used by malingerers trying to get on disability.
Getting on disability is the hardest, most humiliating process. You are out of touch if you think there's a ton of people out there malingering, especially just to get on disability.
Most doctors don't care to know, they just want to diagnose and be done with it. Modern research suggests issues with nervous system likely caused by changes in processing either due to nerve damage or some other chemical component.