Question

Why planets are spherical or geoid in shape?

Answer 1

The shape of the planets are always spherical because of the force of gravitation. The gravity pulls the surface materials into the center, and gravity always act along the centers of the body. When the planets are formed they are in molten state, and the gravity pulls the molten materials towards center. and when it is cooled the planet remains spherical only
But the shape is not perfectly spherical, it is geoid because of their rotation around their own axis, this spin causes somewhat disturbance in the shape of the planet.

Answer 2

All planets are round because of gravity. When our solar system was forming, the force of collision of gases and dusts caused the newly forming planets to become hot and molten. The force of gravity pulled this molten materials inward. Later when they cooled, they stayed spherical.