They are scared because when your in the dark you begin to think its like silent. Then, you start thinking about like nightmares or scary things for no reason. I experience that several times. but, that's just my reason. everyone has different reasons.
psychological, all in your head. I believe that we as people, as far as we have gotten into civilization and technology, we have summoned this belief like an ego into our brains as we grow up. Think aobut it, If it werent for the television to have become so commercial, (movie previews, advertisments) we would not see the made up ghosts that haunt places that are very dark. We wouldnt also see these movies that have to do with people being posessed or killed by ghosts or poltergeists. Ever notice the scary movies are dark, as in they always take place in a dark home with no lights or at night time. Also, these previews show the best, scariest parts, which are darker, scarier, not much lighting. Over time, people have developed these beliefs to associate darkness with harm or at least just being scared...For no reason! I suggest that everyone simply stand in the dark and do nothing for some time. Do this daily and you realise there is nothing there, nothing happening, just the fact that there is no light bouncing off of objects with color to show you color through your eyes. It shouldnt be any scarier than with the lights on.