[ She would have seen his dramatic eye roll and throwing up his hands...if not for the fact that the video was attached to his wrist so it was all just a blur. He righted it quickly and rolled his eyes again just for proper effect. ]

Because they can't. A human becoming a demon is as absurd as a dog becoming human. Demons aren't evil, and they're not made from evil. No more than a tiger or a bear is evil. They're basically predators only it's guilt and suffering they feed on rather than blood and meat. It's really that simple.