Gyms make money by catering to what the clients want. School gyms don't care what the clients want, because the clients are a captive audience. School gyms give kids what the parents and coaches want for the kids.

Suppose that instead of paying to go to the gym voluntarily, all adults were made to go to the gym, and all adult competitive sports practitioners were selected from among gym attendees and encouraged to get into sports based on their gym performance. Gym activities would tend to become highly sports oriented, even if the nominal reason for mandatory gym attendance was to promote fitness.