Question

why the earth in egg shape

Answer 1

During the formation of the earth, it was in a molten state. The centrifugal force due to the axial rotating of the earth was greater than the cohesive force. Since the centrifugal force is maximum at the equator and decreases with latitude becoming minimum at the poles, the earth bulged out at the equator. You know this is a very good example of attaining mechanical equilibrium  i.e. No unbalanced forces or torques will be acting on the system.

Answer 2

Earth does not has the shape of an egg but it does has the shape of an orange. It is due to the continous rotation of the earth on its tilted axis