As someone who wrote large parts of the docs for a project: The reason that I imagine few people do it is because if you get involved with the documentation, you will be on the hook for tracking the library closely and keeping the documentation up to date; there will be an implicit, external pressure that the documentation continues working.

That becomes a lot of work very quickly, so I tend to only get involved in libraries that will quickly hit some kind of stable maintenance mode or in which I have personal stake and thus just need to polish up my own notes for how to get off the ground.