Demons are useless: they succumb when a priest yells "I command you in the name of god". Yea, a demon is supposed to be scared of that. I think that kind of yelling from Mom stopped working on me when I was older then 8 or 9. A demon, who's supposed to be more powerful than I am, is scared of that? Beats me. Not only they are useless, they are stupid - why not just kill the exorcist immediately whilst waiting to be spritzed with holy water?
I believe a lot of unanswered phenomenas are the result of our brain and evolution. Some people's brains are better and some aren't. Some short-circuit more easily, some are smarter. Stop thinking we're so special - we're just apes with lots of Nairs used to wax off our hair.