If everyone publicly said that the models can only do things that humans have already done, but you know they can do more, wouldn’t you want to show them otherwise?

Math ability also helps with other things like making models more efficient.

It's one of the few areas where you can verify results. That fits nicely into training models. They aren't just making judgement calls on what would be nice, it's "what can we do?".

Because it's a tool in search of a use case (or many use cases) and mathematics is the most natural use case for it. Mathematics is by definition the art of putting words on a page in a rigorously defined "correct manner" (i.e. in the form of a valid logical argument, a proof) and all LLMs do is put words on pages and evaluating if they're good words is by far easiest when there is a strict definition of right and wrong.