This question more properly belongs in philosophy. However as a student of science the question of why may be addressed as a theoretical question of how. Consider the fact that the earth is traveling in an orbit, and rotating on its axis and our whole galaxy is apparently moving through space. The sphere produces the least amount of surface friction, and the angular moment of rotation tends to keep all particles equidistant from the center. Apparently there are other shapes in our universe, but round is the one we live on and is the object of our study. Perhaps the gravitational attraction between the masses that make up our planet tend to pull everything toward the center, this would suggest that the massive dense center would produce a round surface. This argument would not hold in the development of objects that are cylindrical, rectangular or hollow. I appreciate the question and hope that my suggested answer is helpful and will prompt more investigation into the forces that affect ourplanet.
The shape of the earth is often called a geoid (Earth-like) or an ellipsoid. The rotation of the Earth causes a slight bulge toward the equator. The circumference of the Earth at the equator (24,901.55) is about 41 miles greater than the circumference through the poles (24,859.82 miles. If you were standing on the moon, looking at the Earth, it would be virtually impossible to see the bulge and the Earth would appear to be a perfect sphere.
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