For the same reason some people are afraid of a loaded gun. It can be very dangerous in the wrong hands.

Because so many allow what they see in movies and unconfirmed stories to trick their minds. When dealing with anything you see in movies and what others have told you but you have never seen is to consider what is behind them telling you. Hollywood is going to glamorize every detail to get you hooked on the story line. Some folks will allow the concept to overwhelm them which tricks their minds into seeing the worst possible thing, rather it is real or not. It works on the human fear scale. And can work for anyone if you can hit a nerve with someone about anything. example: I take you to an abandon building and tell you, once inside that there is a rabid dog in there and it is their job and yours to find it. There are no lights and it is dark. Every little noise regardless what makes it your not going to think is anything but the dog. One of two things is going to happen once you get outside. your going to become level headed and think of all the things you could have heard. Or your going to panic and think of everything you swear you heard that was dog related. I am not saying Ouija boards are not to be feared but why they are feared by many.