In general, almost everyone describes Earth as an perfect sphere, but in reality it isn't a perfect sphere.
Is there any specific reason why Earth is not perfect sphere ?
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There is no reason it should be.
There are bound to be irregularities due to local geological differences.
The theory as to why planets tend to be spherical is because any irregularity would cause a non-uniformity in the gravitational field at the surface, and the forces on the surface rocks would ultimately cause it to be spherical. Though, ignoring local irregularities, the Earth has a smaller radius (from its center of mass) at the poles, about 42 km less, than that at equator.
This can be attributed to Earth's rotation. The distance of a point on the Earth's surface located at the equator is larger from the axis of rotation than at any other latitude. As it rotates, this causes a bulge at the equator. The technical term given to this shape is the oblate spheroid.
You can read about the mathematical formula which describes the flattening and more about the formation here.
Shape of Earth is geoid. It's because of the rotation of our planets around its axis. Because of the centrifugal force, the diameter of Earth is bigger at the Equator than at the (physical) North Pole.
Billions of years of rotating have deformed the perfect shape form of Earth - if it has ever had at all.
Planet is not sphere because it is not in thermal equilibrium and is not closed. It internal core is still at higher temperature and it continues receives energy from the sun, as well as the gravity from other planet. This extra energy and external force could results in any shape of planet.
The appropriate question should be "When can we treat the Earth as sphere?"
It is sphere when we look at it outside with at least few radius away from the Earth. The deviation from sphere is of order 100km, while compared with the radius of Earth 6400km, which is only 2%. In particular, the shape still look circle when your telescope resolution is low, and the gravity is very close to a spherical object.
Actually, you can also treat it as a point like object when you are very very far away.
Or you can treat it "flat" (or very rough mountain), when you look at it on the surface when a person stand there.