Because most people play follow the leader. I also don't think you need slin at all and there is no need for GH.
I do agree that bodybuilder is a big game of copy-cat with drugs, training and diet but you cant dispute that mg for mg nothing beats test when you evaluate it based on:
1) ability to pack on muscle mass in a relatively short duration
2) overall weight gain ability
3) muscle mass retention in a major caloric deficit
4) relatively mild side effects
5) ability to run it for extremely long durations without health issues
6) metabolic increase
There are very few other drugs that you can stay huge on, keep making progress, keep staying lean while shoveling food and have relatively okay bloodwork and be able to stay on FOREVER.
I cant agree that there is no need for slin or GH...does it change physiques and arguably make them uglier? Yes. Is it necessary to be as big as possible? Yes.