Because they obviously do? Unless you want me to scientifically prove it which unfortunately I can't do. Sorry.
Oh ok, I thought there was some revelation about fighters being on PEDs that I hadn't heard about.
I think there are far less fighters on it than some may suspect, as the cost benefit in terms of financial loss is far more in favor of fighters avoiding it. Nobody can afford to be out of the sport for a year or more just for a slight gain in cardio. Therefore now, especially with the more randomly stringent protocols from USADA, it just wouldn't make much sense for fighters to risk their careers over something like this.