The Magratheans have been asleep for a very very long time. During that time, their world, once famed throughout the galaxy (""The fires of dawn...!" breathed Zaphod. "The twin suns of Soulianis and Rahm...!") has largely been scoured into a lifeless waste by the elements.
More of the planet was unfolding beneath them as the Heart of Gold
streaked along its orbital path. The suns now stood high in the black
sky, the pyrotechnics of dawn were over, and the surface of the planet
appeared bleak and forbidding in the common light of day-grey, dusty
and only dimly contoured. It looked dead and cold as a crypt. From
time to time promising features would appear on the distant
horizon-ravines, maybe mountains, maybe even cities-but as they
approached the lines would soften and blur into anonymity and nothing
would transpire. The planet's surface was blurred by time, by the slow
movement of the thin stagnant air that had crept across it for century
Clearly, it was very very old.
It's noteworthy that when Zaphod et al. arrive, only a very small proportion of the Magrathean population have come out of hibernation in order to do a quick custom-build from an earlier set of plans. Once the main bulk of the Magrathean population wake up and see what's happened to the surface of their world, they'd presumably set about bringing Magrathea back to its former glories.