why the earth is round?

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.