Compiler are usually deterministic in their function. They use algorithms designed for their purpose, tested to work on many inputs, and integration testing of many programs. They don't hallucinate or do probabilistic things in most cases. They're also much faster than LLM's, especially if parallelized.

I could see LLM's being used to generate compiler configurations or source annotations to improve compiler behavior. They might be good at code generation for prototyping changes to an existing compiler. They're too unreliable and resource heavy to be a good alternative in most cases, though.