Everyone is pretty expensive when they have to live in Silicon Valley - even if not, anyone who interacts with engineers becomes nearly as expensive due to cost disease.

There are other factors preventing improving it, because documenting a new project is effectively adding more people to the project team (even if they're "just" writers) and they still need to learn it by talking to the developers, who might not have the time, so it's not scalable. Third-party documentation scales by not being able to talk to those people and instead reverse engineering everything, but that would lead to embarrassing incorrectness if it was the first-party approach.