You need a good frame of reference to see the athleticism on the high school football and basketball teams vs. the athleticism of the top 25 ranked sectional players. You had to play or coach football or basketball. The top 25 sectional players are good tennis players but are not, as a whole, elite athletes. There may be 1 or 2 exceptions. Conversely, the athletes at the skill positions in football and virtually all the basketball players are elite athletes- much better than the tennis players. For tennis players to get to the pro level they must be blessed with superior one in a million athleticism.
80% of the top American junior boys just don't have it. It doesn't matter how hard they train.