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You don't realize how much fakery is used in 3D graphics. A game with an overhead view has probably assumed in many places that it's going to be an overhead view and so players can never see anything from below. The artwork of what things look like from below thus doesn't even need to exist. Have the camera try to draw a view of looking up at the sky and all sorts of things are likely to look horribly broken. And never mind that the artwork to draw the sky itself surely doesn't even exist.