^^THIS
At around age 13-14 or so, teens NEED to separate from their parents. Themes of the wicked stepmother, the hidden royal birth, the sudden access to magical powers all speak to this innate need. Teens, especially females, it seems, are drawn to reams of this sort of material, as evidenced by the YA fantasy novels that always incorporate the same memes. Males seek out stories of heroism, romantic sacrifice that wins the desirable female,successful quests and universe-changing invention, but they are responding to similar needs to separate from their parents, escape the emasculating feminine and reclaim masculine domination from their aging fathers.
By 18+, into the 30s or even later, the meme is the perfect mate. Enter the romance novel or the chick flick. A mash-up with the earlier themes gives birth to the erotic fantasy genre. There are sub-genres: werewolves (-bears, -cats, -dragons) and, of course, the fantastically popular vampires. Boys graduate to adventure stories, military themes of conquest and domination.
Blame human psychology. Disney and the successful authors/publishers are responding to a perennial niche market. The original Brothers Grimm compilations of ancient folklore were devoted to the same themes, culturally appropriate for their time.
The basic story is: fear, pain (and more fear and pain), then triumphant catharsis. We humans crave these stories, so someone will always provide them.
Greek mythology and Greek drama (Oedipus/Antigone) enjoy these themes as well and are generally better written than zombie novels on the same theme! But try to get a teen to read them.
This is also seen in the “heroic mythos” cycle, or whetever they’re calling it these days, Star Wars, and Harry Potter.
Nobody’s going to read or go to see “The Adventures of Pajama Boy and the ObamaCare Holiday Discussion”
Although it might make a great parody movie. :P