Ohio alone has over 1,000 high school marching bands. There are over 20,000 high school marching bands in the United States. There are also several hundred college marching bands to add to the mix.
DCI audition camps, thanks to these marching bansd, have thousands of kids turning out every year for both World and Open class.
The problem with marching bands tied to high schools is that most of those bands don't really represent much of an opportunity for the kids. Most of those bands march out onto the field, play a tune, move some, play another tune, move a little more, and call that a show. Along with a couple of parades, they're technically a marching band, but barely. There may not be much interest by the kids or the parents or the school in building a better program than that. But if there are opportunities outside marching band for the kids who do have an interest in going beyond the most minimal definition of "marching band", then that's good for the marching arts and good for those kids.