To monopolize basketball talent for the teams in those leagues; and in the case of the NCAA, extract most of the value provided by the players for the benefit of the teams, their coaches, and other well-paid staff.
The NCAA establishes rules and regulations regarding recruiting and competing for collegiate athletics. The NBA similarly is a business association of the owners of NBA teams. The "one and done" rule is a rule imposed by the head of the NBA, and the subject of "one and dones" became a larger debate in 2005, when the NBA and its players added an age limit that required players entering the draft to be 19 years old or have completed their freshman year of college. As a result, players who would have jumped directly to the pros out of high school were forced to spend a year in college before entering the draft. In my opinion, it has made the March Madness completely unpredictable, as elite programs can't be predicted from a prior years performance.