The Satanic Panic was around long before D&D and heavy metal music and it never really left. It just moved on to new targets. Someone, somewhere has always been doing something satanic as a plot, it's just easier to find people talking about it in the age of social media.

Same with conspiracy theories. The alt-right never invented anything new, they just recycled things that wingnuts and cranks have been spouting for centuries and adapted them to modern settings. Look at the parallels between "Hillary tortures children and drinks their blood to stay young" and the old Blood Libel conspiracy for example.

I go back and forth as to why I think this kind of thing is so enduring. Either people need to believe in some kind of evil because it allows them to feel good about being on the "right" side of something. It could make them feel smart to find "secrets" that people won't openly admit (because they aren't true). Or they feel some kind of cognitive dissonance if they don't have an enemy and invent one to quiet it. Or maybe it's just a convoluted effort to take control over something they aren't comfortable with in a "he who can destroy a thing, controls the thing" sort of way. Maybe all of them even.

I dunno, like you said, people are goddamn stupid and fucking nuts sometimes. Shame they're so often dangerous because of it.