Many times the only documentation available is cornering one of the engineers and grilling them. I've seen cases where the official documentation is a few neatly typed pages, with handwritten corrections by several hands and no explanation whatsoever.

It is just as with our software: Many times there is (next to) no documentation on the inner workings available, developers find writing such boring (and it is suprisingly difficult to explain what you know inside-out to strangers that can't ask back). Sure, you can eventually figure out C code by staring at it long enough, while staring at your average device won't help; but the forces at work are much the same.