From what I've read and listened to in interviews, a lot of the PEDs including HGH aren't used to get the person yoked or swole, but because it pushes endurance and promotes healing, allows for longer training. Come fight night they cycle off and are technically "clean" fighters so they pass the tests. The benefits are in the training and extra time put in the gym, and less about the muscles.
Simple math between two identical fighters. Fighter A puts in a clean training camp with a base of 100. Fighter B is dirty as fuck and juices, puts in 115 the training that fighter A got. That additional 15 percent ends up being the difference in the fight. Totally unfair and difficult to track unless they do more random in-camp tests a la Overeem during the horse meat era.