I wonder is this because most humans can't find joy in the merely real, praising deities and trusting in other supernatural stuff like signs and horoscopes, so disbelieving and living in reality is abnormal?

Well, there is a group of people who are SlateStarCodex readers, but aren't LessWrong readers. Some of them say that they do not find LessWrong topics interesting, they are often too speculative for them, and they prefer SSC because they want to discuss things like policy and economics. I think that this explanation is applicable in general case as well - most LessWrong topics simply aren't that interesting to most people and they prefer to spend their time on other websites.