That's like saying in football, if being a striker is so difficult why are 3/4 goals scored by strikers. It's a role in the sport and a major one.
Because scoring goals is what the game of football is about and how you win matches, that is what the team is always working towards. Sprinting has little to no impact on the outcome of a Grand Tour(other than crashes) and is a sideshow in between what really matters.