This would force every athlete who wants to compete at a high level to be drugged up to the gills or not even be able to qualify versus doping users. A lot of these substances cause negative long-term health issues, too. Also, every sport'd become a competition of the best chemical / medical team / available money instead of the best athletes.

People can do w/e they want with their bodies on their own, but if they want to participate in an organized competition they need to adhere to the standards set that are meant to protect every other participant and the integrity of the sport.