I've always wondered this and I think I know the answer. For most believing in God starts at a young age. You are young and can't think for yourself so if someone molds your mind to believe in all sorts of different things from a very young age it will be fairly difficult to undo the brainwashing. Take catholics for instance. Most catholics were baptised before they could talk and received the most important sacraments of the eucharist and reconciliation at a relatively young age. Then most are confirmed before they are out of highschool. OK now let's think of a theoretical situation. You are an educated adult over the age of 20 and somehow you have not been exposed to any religion. One day someone walks in your house and tells you summaries of the bible stories. What would happen next? You would laugh your ass off and tell the person you would rightfully think is a lunatic to get out of your house. If religion was introduced at later stages in life there is no way that it could flourish because no one would believe it.