Probably because you and your boss have valuable context and historical knowledge about why things are the way they are, and this is of course lacking when you pawn the task off to Copilot.

Yes, this right here. Generated content that's based on sources that readers already have access to has no value beyond what they can ask an LLM to generate themselves.

Now, if you write it with tender loving care and bake all that stuff into it, then will humans read it directly? Maybe not, but the information that their agents read and then pass on to them at least has a better chance of being correct and useful. If the success criteria for your documentation was "measurable count of human eyeballs" and not "better outcomes", your docs aren't going to be good anyway.