Documentation is never good enough
If there is too little documentation or badly phrased, people will not find the answer there.
If there is too much written on a subject people will not find the answer since there is too much written and they are only looking for the answer to their tiny, tiny question.
If there is a FAQ, the exact question the user is looking for is not answered there.
Heck, even if the documentation is perfect (in your mind), lots of people will just mail and ask anyway since they believe that is a faster way to get the accurate answer. Or they do not believe the documentation. Or they for some reason decide that the perfect online documentation must be out-of-date.