Imagine a scenario where 90% of athletes cheat, effectively bringing their performances beyond what the human body can naturally achieve. The "natural athletes" won't stand a chance, so we're unlikely to ever memorize their names.
On the other hand, assuming that only 1% of the athletes use drugs, using them is that much more effective, having the capability to turn an already good athlete into a super athlete, which makes taking the risk all the more worthwhile. The likelihood of a good percent of the winners being drug users thus remains pretty high IMHO.