Ok, if whether its GH Insulin or mom's meat loaf, why don't comptitive bodybuilders just stop doing it if it ruins their physiques?
Because sadly most bodybuilders work on a "monkey see monkey do" system. Instead of putting their own sense of craft/aesthetics/values into the way they approach their physique, presentation, they simply opt to do what everyone else says is the "in" thing. One person raises the bar/standard and the rest follow.
In the case of Slin... First there was Duchaine, which was the real pioneer of its use in Pro bodybuilding...Dorian was the first to really grasp its potential, he ran with it, but he was also pragmatic enough to make it work for him instead of be a detriment. Others followed suit, trying to chase the size game and be competitive with Dorian, and instead of succeeding, many ruined their physiques, or turned to synthol to compensate.
When I was last pushing in Venice...Slin was one of the most popular items on the black market...even by normal everyday lifters who had no competitive ambitions...simply because they heard others were doing it. The amount of stupidity, and lack of individuality in this subculture is staggering, but it is just the way it is.