The reason scam guru's can exist is answered in the first part of your sentence.
Sometimes gurus are just... right. Years ago, there was one in the Philly area who attracted some attention. One day a couple of friends of mine were driving back from work when they caught an interview he gave. The interviewer asked, among many other things, why, if he'd discovered the way to Truth, he didn't have more followers.
He replied, "Because most people have shit for brains."