Here's why:

* Rotation: The Earth's rotation causes centrifugal force, which pushes outward on the planet's mass. This force is strongest at the equator, causing the Earth to bulge outward.

* Gravity: Gravity pulls everything towards the center of the Earth. However, due to the centrifugal force, gravity is slightly weaker at the equator than at the poles. This contributes to the bulge.

The difference between the Earth's equatorial and polar diameters is only about 43 kilometers (27 miles). This is a tiny difference compared to the Earth's overall diameter of roughly 12,742 kilometers (7,926 miles). So, while the Earth is not perfectly spherical, it's much closer to a sphere than a pear!