For basically the same reason every programmer worth 6 figures doesn't use the same keyboard. Money clearly isn't the limiting factor, neither is not wanting the best tool for the job. "Best tool for the job" though, is subjective; and by the time someone has put in the thousands of hours of practice to excel at their craft, they will undoubtedly have preferences regarding the minutiae.
Tennis players can't choose their brand of tennis balls either. So it's only a small step to enforce the type of racket or any equipment which can cast doubt on fairness.
