Because drag increases at the cube of velocity. So improvements in top speed require huge increases in power. However in performance improvements short of top speed increased power increses climb rate,

acceleration rate, and shortens takeoff run. Top speed, not so much.

 

Lynn E. Hanover

 

The less obvious point is that since the drag determines he top speed for any amount of power, the payoff for carefull drag reduction is higher top speed at much reduced cost. And higher speed for any throttle setting.

 

Lynn E. Hanover