Because all human beings are subject to mistakes, cheating, illsuion and imperfect senses. They’ll forget stuff, trust the wrong people, slip up, miss things, inadvertantly reveal things, etc.
And the more evil people are, the more they are subject to these imperfections.
Their own “cleverness” will be their undoing. Eventually.
So why don’t all those imperfections you cite prevent people like you from “catching” them?
Not that I agree with your fairytale notions. I just wonder how you defend postulates that are inherently indefensible.