Well, we don't really know if they aren't doing exactly that for their internal code repos, right?

Conceptually, there is no difference between fine-tuning the LLM for being a law expert of specific country and fine-tuning the LLM for being an expert for given codebase. Former is already happening and is public. Latter is not yet public but I believe it is happening.

Reason why big co are pursuing generic LLMs is because they serve as a foundation for basically any other derivative and domain-specific work.