Like many things in complex projects such as operating systems, accepting PRs is not as simple as "if this one use of a tool works, why not accept it?"

Why? Because the onus of determining fitness of purpose and correctness has to be done by someone. So if a PR is buggy and/or lacking in test coverage, but was made by a contributor making an honest effort, then experienced contributors can help educate the person as to what is missing or needs to be considered. Ultimately, this is a form of apprenticeship.

Contrast that with someone using an LLM to produce the exact same PR. Who can be mentored? Who can be referred to materials which can deepen requisite knowledge?