Free and Open Source Software

I'd say the documentation problem is even worse in closed source internal stuff at companies. You often only have internal users (ie your colleagues) so instead of properly documenting, often people just tell stuff mouth-to-mouth (tribal knowledge basically).

Also, you rarely gain any points for writing good documentation at jobs because you have to spend time doing it making the coding itself slower. And some people deliberately don't write docs cause they think it'll provide better job security.