The boring (and probably mostly correct) answer is that engineers don't like writing documentation so it is the last thing to get done, if it gets done at all.

The boring (and probably mostly correct) answer is that engineers don't like writing documentation so it is the last thing to get done, if it gets done at all. This is especially true of FOSS projects. You may be motivated to spend your free time hacking on something cool and interesting, but spending your free time writing documentation is another matter.