I think it has to do with the fact that we all are cognitive misers. Just think about the comments that this post has generated. Do you really think that every commenter has carefully thought about the subject before articulating an answer? Everyone here is probably forwarding his own pet theory, myself including.

We all probably carry a lot of inaccurate beliefs with us and never bother to check them. Reevaluating our beliefs costs time and effort, keeping the status quo in our mind is effortless. So we only reevaluate when there are strong reasons to do so. As long as people can get away with a belief without being socially reprimanded for it, most will do so. Why bother, since we all probably have more stuff on our todo list than we can manage?