Just speculating on news reports I read. Steroids do enhance muscle mass, again, to my limited understanding. And muscle helps make those long drives necessary to a low golf score.
Timing and technique are far more important than muscle mass in golf. In a game that is largely mental, taking drugs that increase mood swings and agression would not be wise. On a side note, I do not like Tiger. He is getting what he deserves.
It is not really all that necessary for long drives. Where strength really comes into play is hitting it out of thick rough and bad sand trap lies..