That's one of the joys of fiction: building a world entirely from scratch.
But regardless of how much backstory we get, all dystopian fiction b3egins, at its essence, with the end of civilization as we know it. Which is why I found this quotation from Camus ironic.
"The purpose of a writer is to keep civilization from destroying itself." It's ironic that the best dystopian fiction, by destroying civilization on paper, helps us keep civilization together in the real world. Dystopian fiction — and all good fiction — helps us keep a level head and a wider perspective. It allows us to see arguments from others' perspectives, and in so doing, it helps us become better human beings.
Think about the works of fiction that have had a real and positive effect on the world — The Grapes of Wrath, The Jungle, 1984. Think of works of fiction that are evoked in arguments about real-life controversies — The Handmaid's Tale, Atlas Shrugged, Slaughterhouse Five.
This is what makes storytelling worthwhile, and why we must be vigilant against forces that that want to curb our right and ability to tell stories. Literature isn't just entertainment, and I shiver to think of what our world would become without it.