That’s primarily because the contribution process to official documentation is still fairly high-friction and no one seemed to have bothered to go through the pain on that particular topic. Especially for the NixOS manual there are no dedicated maintainers who will have the time to review pull requests. @jtojnar does a great job already, but like any volunteer will be time-constrained and fighting against a flood of notifications. We’ve done a few things to make meaningful contributions easier, but it’s a long way to go. The call for documentation maintainers is still relevant.
It’s not possible. If you’re willing to go that far you may as well contribute to official docs. We’d be glad to have you around the documentation team if you want to help out!