because if you were really good at a sport, you would fucking love it. its just human nature to think that things we are really good at, especially were we can prove we are better than someone else, are really fun. Plus some of the social benefits from playing a sport are pretty awesome.
But, until i went off to college i had no idea how big marching band was in some areas. Like, it was a bigger deal to be in band than it was to be a starter on varsity...given their football team was awful.
It varies from area, to area, but in general band will always be looked down upon as the geeky halftime show that no one cares about.