Why would that be desirable?

If we take the human brain as an example, it's pretty bad at computation. Multiply two 10-digit numbers takes forever, despite the enormous size of its neural network. It's not the right tool for the job - a few deterministic logic gates could do that much more efficiently. That same circuit can't do much else, but multiplying, oh boy, it's good at that! Why do we think that artificial neural nets would be the right tool for that job? What's wrong with letting the LLM reach out to an ALU to do the calculation, just like a human would do? It's surely going to be quicker and require less energy.