Arrakis is hardly just a monocosm. It has a living breathing ecological history that explains why its the way it is rather than it just being an arbitrary set piece (like most of the monocosms in say, Star Wars). Originally it was a lush tropical realm until the advent of the sandworms who in turn devastated the environment causing it to become the wasteland it was at the time of the first Dune book. As the series progresses, the environment of Arrakis changes as well and I believe by the end of Frank Herbert's books large swaths of it had returned to a thick, vegetative state. The changing nature of the planet Arrakis is key to understanding the themes of the Dune series.