Because it takes time and effort to write documentation.
If people __can__ actually read undocumented code with the help of LLMs, why do you need human-written documentation really?
Docs are a form of error correcting coding for code. Docs+code allows you to spot discrepancies and ask which one is the intended behavior.
It doesn't need to be written by a human only, but I think generating it once and distributing it with source code is more efficient. Developers can correct errors in the generated documentation, which then can be used by humans and LLMs.