Because the dimples maximize the distance golf balls travel. Dimpled balls travel up to four times farther than smooth-surfaced golf balls.
In the early days of golf, smooth-surfaced balls were used until golfers discovered that old, bumpy balls traveled longer distances. The science of aerodynamics helps explain the dimpled phenomenon. The dimples reduce the drag on a golf ball by redirecting more air pressure behind the golf ball rather than in front of it. The higher levels of pressure behind the golf balls force them to go far distances.
The dimples change the levels of pressure by bringing the main air stream very close to the surface of the golf ball. The dimples, or "turbulators," increase the turbulence in the layer of air located next to the surface of the ball. This high-speed air stream near the ball increases the amount of pressure behind the ball-thereby forcing the ball to travel farther.