Tradition. Math as we do it goes back to the original Academy, the one Plato started. For most of its history, math was all about the mathematician, you studied math because studying math made you better. We didn't even start thinking about applications in math education until the industrial revolution meant we needed factory workers who knew a little math and it was just assumed that you didn't want to try to teach the proles how to think. Math academia is weird to me. In physics, researchers insist that there are applications to even esoteric problems, often to a fault. Mathematicians,on the other hand, often get offended at the idea that math needs to be created to solve other problems, that rather, math is done because it is elegant and beautiful, and if it happens to be useful to someone outside math that's just an unintended side effect.