People in all sorts of different positions take performance enhancing drugs. We as a society tend to focus on athletes for some reason. How many of the programmers you work with are taking some kind of stimulant or nootropic? Everyone is looking for an edge. How many famous CEOS "cheated" at some point to get to where they are?
Going along with your postulation outright, I'd note that most of the drugs consumed by musicians wouldn't be called "performance enhancing" in the context of general terminology - alcohol, grass, hallucinogens, downers, stimulants - pretty much mess with mental capacity and function, not really "enhance" them. A couple cases can be made for certain uppers, but I kind of look at the relationship of drugs and musicians as one rooted in self-medication moreso than performance related. I have heard of orchestra/symphony types using beta blockers.
Because music isn't a competition. It's a creative endeavor.
When musicians compete for positions in symphonies it is a competition.
It's not as if sports don't demand creativity.
Life is a competition. There is definitely competition in music. People battle for spots in orchestras. People battle for commercial dollars, etc.
Because we don't have a lot of rules for musicians. Of the rules we do have, they are a social contract rather than a binding rules committee. And of those rules, none of them govern drugs. Plagiarism, yes, but drugs, no.
Music isn't, in general, a head-to-head contest between individuals or teams with an objectively determinable way of keeping score.
Money: it keeps score so you don't have to.
Why are fans of athletes allowed to drink and beer companies allowed to advertise with obvious sports oriented themes in their ads?
Do musicians actually use PEDs? How would they help?
They could help with stage fright or the stresses of touring.