Um its supposed to be a PLANET! Do you realise how big a planet is? Even from the movies you would only see an extremely small portion of one planet. The rest of a planet could be anything at all. Its completely in the hands of a game/art/environmental design team to come up with ideas on how different zones could work planet. You could have desert, rain forest, jungle, ruins, industrial, swamp ... absolutely anything the 'creative' team could come up with.
To say one entire planet must have the same terrain throughout is absurd.