Extreme weather with atmospheric acidification that slowly eats away at everything on the surface like a polishing a rock. It would take hundreds of thousands or millions of years but it's plausible as the sun slowly expands, which causes tidal stresses and significantly increases surface temperatures making the landscape a lot more dynamic.
Dunno. Also, the author hints than the night is endless and cold and the child, which the author uses to convey that part of the story, is scolded by his parents and ordered to get back underground where it's warmer.
Hmm... maybe it only looks smooth from space, like Venus or Uranus? An impenetrable cloud layer (like in The Matrix) would explain the cold on the surface and the endless night.