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.