That's an excellent point. A lot of the reasons for taking steroids and other enhancers among athletes is to help recover from injuries faster and to prolong their career, not necessarily for the bulging muscles and extra strength that comes with it.
But then why haven’t the rest of the tour tried that route
As for that, maybe they realize that Tiger is like the basketball superstar who hardly ever gets called for fouls in spite of knocking players to the ground or taking extra steps with the ball, while the non-superstars get called for any little contact they make against the superstar. In other words, the officials may be willing to look the other way in the case of Tiger but may not be willing to do so with anyone else.
Still, i'm not convinced Tiger took anything illegal, but i can see why the suspicions are there.