Why Are High School Players Not Allowed To Turn Pro Immediately After High School?

Some of the greatest NBA talent, present and past, have successfully transitioned from high school to the NBA. With the success of high school players like Kobe Bryant, Dwight Howard, Lebron James, and Kevin Garnett how can you justify denying others the same opportunity. Better yet, why would you deny anyone the opportunity to earn a living for themselves doing something that they are fully qualified to do.