Actually, your premise isn't correct. Earth and all the other significant celestial bodies are round (or more specifically, oblate spheroids) due to gravity. As they formed, gravity pulled them together collapsing them into the most compact shape possible, which is a sphere. The rotation of these bodies, Earth included, can cause a slight flattening at the poles and bulging at the equator, making them oblate spheroids rather than perfect spheres. This is a physics/astronomy query rather than a programming one, but I hoped it answered your question.