I can't speak for other people, but personally I do value documentation. Not that I want to spend all my time documenting someone else's work, but when I need to learn about something, documentation is invaluable. No, it isn't as fun as writing code, but that doesn't make it useless. If someone else wants to contribute to FOSS and isn't a coder, but can do tech writing, I for one would appreciate their contribution to documentation.
I value documentation as well.
The problem is that the people changing the code out from under the documentation, so that the documentation quickly becomes out of date, or, worse, incorrect and misleading, is those people who are doing that to the code *not appreciating* the documentation effort.
At worst, there needs to be an agreement that things will stay the same for a while, or for at least a major version number, before the documentation goes out of date. And as you've noted with git: when things grow organically and incrementally, it's going to be near impossible to keep the docs in lock-step with the code -- particularly if the only way to make them match up is reverse engineering the code until you know enough about it to document it accurately and completely.
At one point in time, I wrote a rather complete internals book on FreeBSD; but the OS changed out from under the book too quickly, and so it was inaccurate, except for a particular major revision. And even then, there were sufficient differences even in the point releases (to the odd minor version number) that, unless I'd included a CDROM set or DVD with the book itself, there was no way that it was going to be useful for its intended purpose as a college textbook.
So yeah, documentation would be nice, but it's only going to get there as a divided labor effort if we agree to write design documents up front, and then follow a cathedral model for both the docs and the code that come out of those designs.
I think one of the major problems is that when you make something understandable by documenting it ... it makes it a whole lot easier for someone to step in and know how to "improve" things, until the docs are out of date again. At least, that has been my personal experience.