All planets are round because of gravity. When our solar system was forming, gravity gathered billions of pieces of gas and dust into clumps which grew larger and larger to become the planets. The force of the collision of these pieces caused the newly forming planets to become hot and molten. The force of gravity pulled this molten material inwards towards the planet's center into the shape of a sphere. A planet's gravity pulls equally from all sides. Gravity pulls from the center to the edges like the spokes of a bicycle wheel. This makes the overall shape of a planet a sphere, which is a three-dimensional circle. Later, when the planets cooled, they stayed spherical.
Planets are not perfectly spherical, though, because they also spin. The spinning force acts against gravity and causes many planets to bulge out more around their equators. Even if the original shape was not spherical, gravity will flatten out the bumps. The bigger a planet (or moon) gets, the stronger its gravity is. Therefore mountains and other high land get squashed under their weight and crumble down to a certain limit in height. Similarly, depressions and craters get filled in.
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