Like many other drugs that have a chance to boost performance, SARMs are banned and prohibited by the World Anti-Doping Agency. The rules that the agency uses to determine this are fairly simple, but SARMS falls under and violates practically each rule.
Not only do SARMs have a proven way of increasing your performance, they also offer several other benefits to those who take them. Things like an increase in endurance or bone strength might not be obvious from the moment a race or a long event starts, but they can make a big difference when everyone in the competition is competing using only their bodies. To change that would be to risk interfering in the process: to allow someone an unfair advantage would be unacceptable.
SARMs are prohibited – and likely will stay banned – by WADA due to all of these reasons and more.