It's easy to answer, and multi-faceted.
- The arts are only appreciated by a few vs the majority. Everyone likes a little art--but it's hard to get everyone together for a common art or cause. You like what you like. And it doesn't matter how good someone draws anime, or paints flowers, if you hate flowers you aren't ever going to appreciate their talent really. There are aspects of football for everyone--it's a formula.. No one gets pumped and psyched for the new poem coming out that week. Or the new painting on the wall. It's a different kind of enjoyment. And in high school, where abilities are measured, and people are generally extroverted, it's something easy to enjoy with others and rally with. You can not be a huge fan of football at all and enjoy high school football games. It's hard to be part of art without having a genuine enjoyment of it. (This is all generally speaking here)
- Physical strength is awesome to watch. Seeing feats of strength is instant, and you can see it. Watching someone paint is like the most boring thing ever unless you're painting yourself. And not everyone can be an artist. So it's way more involved with the audience than art. Sports are entertaining for the masses.
- It pays. Being involved in sports COULD end up making you bank if you make it big. Rarely do artists move on to 6+ figures. Because, like I said, even though we NEED art, and use it everywhere... it's not a high paying field. Music can be, but not the kind of music that is generally taught at school.
- It creates a brotherhood within its own ranks. Artists are generally the loner type.. so they attract loners. Band has a .. sort of.. brotherhood.. but it's full of nerds. Sports create a fight and challenge that people can SEE. No one really understands how difficult a song is to play if they've never picked up an instrument. No one cares it took you 7 months to learn to play wonderwall perfectly on cello. Music and art are 'cool' in their own ways, and difficult no doubt. But the challenge of physical activity is easily witnessed, and seen, and it's got quick rewards for both the players and the audience.
- We as humans on some level crave physical strength because it's necessary for survival still on some level.