I'm gonna be real with you: it was fun. I was never the best athlete or trombone player when I did marching band but myself and many other kids who honestly didn't have a future playing sports at a collegiate level had genuine fun playing and practicing.

And after school is literally the most convenient time to practice for everyone involved including coaches, parents, and students due to the peculiarities of American living. When I was in school around half of American school children played some kind of organized sport and most probably got the same night's sleep as the other half that didn't. At the collegiate level they do what you suggest and often have two practices a day in the morning and evening for the biggest sports.