Well, not everyone who is qualified to write good documentation wants to work on the code, nor would they necessarily be as good at writing code as they are at writing documentation. If you're writing API documentation, you need to be enough of a coder to understand what the engineers are talking about -- and what they aren't talking about but really should be! -- but that doesn't mean you need to be coding at a level where you could have a career writing apps built on that API, let alone coding at a level where you could have a career implementing the API itself.

It's the "culture that rewards and values the work" that's really the issue, I suspect. Apple certainly used to have a culture of good documentation, and I know they've hired good writers, technical and otherwise, over the years. It's actually kind of difficult to explain why their docs are the way they are, other than just not putting any real priority on the necessary work.