"Regulating it" means creating new rules. Which would, once again be broken if athletes thought it would help their performance. And you'd end up at square 1, except everybody who was previously clean was now forced to take all the drugs the new rules permitted.

And there's nothing really arbitrary about the rules, or at least no more arbitrary than almost any other rule you can find. Just because you can argue about the exact requirements to become a lawyer in Iowa doesn't mean that my hamster is as good a lawyer as the top graduate of Harvard Law.