Why accept PR's in this case, if the maintainers themselves can ask their favorite LLM to implement a feature/fix an issue?

Because it might require time consuming testing, iterations, documentation etc.

If everything the maintainer wants can (hypothetically) be one-shotted, then there is no need to accept PR's at all. Just allow forks in case of open source.

Obviously - it takes effort to hone the idea/spec, and it takes time to validate the result. Code being free doesn’t make a kernel patch free, though it would make it cheaper.