It's wonderful that some are rounded up and sold to people who want them, and it was even better back when there was more demand for cheap horses so that more were sold. The misguided meddlers who ended horse slaughter in USA (against the recommendation of every single association of veterinarians) destroyed the market for cheap horses, so it's no surprise that those numbers are lower. There's no guarantee that any particular mustang will make a good riding horse, so fewer people are willing to deal with that risk. The slaughter ban was bad for lots of domesticated horses about 8-10 years ago (one could drive around and see fields full of starving and neglected horses), but that level of owner is largely without horses at this point. If it's too big a risk to keep a few domesticated horses around, it's certainly too big a risk to buy mustangs.

That's only part of this particular issue, however. Even if five times the current numbers were sold to willing owners, feral horses would still be destroying the environment. They aren't native and they are uniquely suited to disrupting the life cycle of native plants. The American hunter is probably not ready to control this population in the way that e.g. deer and elk populations are controlled. The best solution is probably sterilization, but even that would be very difficult at this scale. That is especially the case given the level of interference that could be expected from the Humane Society, ASPCA, etc.