The same reason you don't use your desk calculator to run Python scripts on huge datasets. It's the wrong tool.
Running an LLM and parsing and computing mathematical expressions are entirely disjoint operations. You need highly specialized code for each, it makes just as much sense to put a calculator in your LLM as it does to stuff a Python interpreter in a calculator. Could you? Of course, software is infinitely flexible. Does it make sense to do it? No, it makes more sense to connect two different specialized applications than to try shoehorning one into the other.
