adventure time lore is insane. it starts off just being a goofy kids show that has magic for no reason, but then you learn that all magic users are manic and/or depressed (what betty calls sadness and madness). because it turns out magic is actually a cosmic force beyond mortal comprehension, that itself was learned from cosmic entities that predated the existence of time itself inside a sea of monsters. and "magic" really is just understanding more about the nature of the universe than most people. that, in a way, reality isnt "real" and understanding that allows you to mold it. and thats magic. but that drives you to insanity and/or apathy. and there are beings who hold significant cosmic importance who are more prone to magic. and the reason magic became prevalent on earth is because of a nuclear war a thousand years ago, which released the entity that represents the destruction of all life onto the world. and after a nuclear apocalypse this gave way to a new earth, where magic could thrive. but a lot of the beings we see arent even magical, theyre just mutants from what happened 1000 years ago. and humanity as we know it has been all but wiped out. but everything stays the same because cycles of war and violence continue. and it doesnt matter if its nukes or magic. everything stays the same, but still changes.