It probably also doesn't help that the number of people who look at a given piece of code follows a power law, which means that when you're doing the looking, you are probably looking at a piece of code that many other people have looked at. Everybody remembers the terrible mess that they had to puzzle out without any documentation. Most of the time they don't remember all the little experiments they did that never saw the light of day, where time spent documenting would just be wasted.