“The NBA hasn’t ‘always’ been tied to college sports. As recently as LeBron, players could enter the NBA directly from high school, but now have to wait one year.”
"Darryl Dawkins, Kobe Bryant, LeBron James, and the handful of other players who went to the NBA straight from high school during that era are the exceptions that proved the rule - the rule that the NBA was and still is highly dependent on the colleges for a large majority of its talent development.
Instead of the few high school kids who managed to make it in the NBA, better example of a non-college source of talent for the NBA is the Europeans. A lot more NBA players have come to the NBA out of European pro teams than ever made it straight out of US high schools.