Because martial arts [business] are kids driven.
BS-JJ and K.Maga places are full of kids as well. As a kid, they want to have a similar or same belt as their friends who referred or started early.
If someone doesn't like belt system and no weight classes (LOL), take Kendo.
Probably that is one only martial arts, which they do not have a visual belt. They use kyū/dan system.
https://en.wikipedia.org/wiki/Ky%C5%AB