To my mind, well-written fantasy, science fiction and horror is more "believable" than most examples of capital-L Literature. One can argue that the reason is because the spectacle of the supernatural distracts from what is otherwise one of the three Stories in human thought.
But I would counter by saying that the Supernatural works as an unconscious metaphor for things that are universal to human experience. When written well, the tale of a young mage coming to terms with their power, or a Vampire struggling with his base urges is every bit as effective for relaying the experiences that inspired the tales as a novel set in the world we know as our own and dealing with those things in a more literal (literary) sense.
But I would counter by saying that the Supernatural works as an unconscious metaphor for things that are universal to human experience. When written well, the tale of a young mage coming to terms with their power, or a Vampire struggling with his base urges is every bit as effective for relaying the experiences that inspired the tales as a novel set in the world we know as our own and dealing with those things in a more literal (literary) sense.