They're doing the fun part (coding) for free. It's well-known that developers don't like writing documentation (esp "properly" which is ill-defined and for some definitions can be a much larger task than writing the code in the first place); in fact, does anyone enjoy writing docs as much as an engineer enjoys writing code?
So this makes total sense to me. As a user of free software, you can choose: a) pay for a commercial package with comes with docs; b) use free software, and pay for the docs in the form of a book (perhaps waiting a few years until lots of people want to do the same); c) use free software and figure it out from breadcrumbs the developer left in whatever forums they use; d) (c) but then step up and write some docs.
Too many people think that free/libre software is supposed to be like commercial software, only better, but it's not the case. It's definitely better in the "libre" way, and of course cheaper in the "price" way, but it's usually not packaged for easy consumption, nor is the developer going to do a bunch of work they don't enjoy for other people who don't want to pay them for it.