This is old hat. I had the thought and then looked it up and there are plenty of papers explaining why athletes would dope even given anti-doping rules. Here's one.
1. The number of individuals capable of winning an event are small.
2. If Joe wants to be competitive he either has to be one of these people, or dope.
3. If Joe does not dope he will not even be competitive, and he will not gain sponsorship (the importance of this element might be a new paper) and will not be able to afford to compete.
4. Even if Joe does not win, he still gets to compete when he dopes.
5. If Joe does not win he is less likely to be scrutinized for doping.
6. Because Joe is doping, the capable athletes face greater competition.
7. Some of these athletes may feel threatened enough to fool around with doping.
8. These are the guys who get caught. (This also might be a paper. How many of the athletes caught doping, really are the best at what they do - like Barry Bonds -and they only dope because of the artificial pressure they face from mediocre dopers.)
9. There are obvious game theory implications to these questions.