I believe it's because having a foundational understanding of maths and logic is important when solving other problems, and if you are looking to create an AI that can generally solve all problems it should probably have some intuitive understanding of maths too.
i.e. if we want an LLM to be able to solve unsolved theorems in the future, this requires a level of understanding of maths that is more than 'teach it to use a calculator'.
More broadly, I can imagine a world where LLM training is a bit more 'interactive' - right now if you ask it to play a game of chess with you it fails, but it has only ever read about chess and past games and guesses the next token based on that. What if it could actually play a game of chess - would it get a deeper appreciation for the game? How would this change it's internal model for other questions (e.g. would this make it answer better at questions about other games, or even game theory?)