The earth is slightly flattened from a perfect spherical shape because of

Because of the effects of the earth rotation

Explanation:

The earth's rotation causes an outward force which is minimum at the poles and maximum at the equator. This forces causes the earth to be slightly flattened.

However, this effect is very little as the diameter at the equator and poles are approximately 13000 km. The degree of flattening is just about 0.3%