1) It's terrible for ethical reasons to pump people with drugs and especially terrible for teenagers. The drugs can have serious side effects and making it legal could have terrible lasting effects.
2) You could force talented players that react badly or can't take drugs out of the game, the game becoming a case of who can tolerate the most juicing in their system. It could easily create a game where you can't be a professional without serious doping.
3) Could create a scenario where the best drugs win, turning footballers more like F1 cars and doctors into the key person in the operation. Don't think anyone wants that.
4) Does football really need to get more physical focused than it already is at the expense of technique? It wouldn't lead to a better spectacle.
It goes against the fundamentals of what is regarded as sport, the logic is flawed, it's usually associated with 'oh there are definitely teams at it and get away with it (unproven), so they should just let every team at it'. By the same logic, why even have police, since people get away with crimes all the time? People get away with stealing cars, so why not let all criminals steal cars etc