zlacker

> Anything critical and commonly used is pretty well documented

I'd argue the vast majority of software development is neither critical nor commonly used. Anecdotal, but I've written documentation and never got any feedback on it (whether it's good or bad), which implies it's not read or the quality doesn't matter.

>>Cthulh+(OP)
Sometimes the code, if written cleanly, is trivial enough for anyone with a foundation in the domain so it can act like the documentation. And sometimes, only the usage is important, not the implementation (manual pages). And some other times, the documentation are the sandards (file formats and communication protocols). So I can get why no one took the effort to compile a documentation manual.