All the planets in our cosmos are also round, but in the cube shape or in the pyramidal shape, or in some other irregular shape, the Gravity. The gravitational force of those planets is also equally inward from all sides. This is a round shape that comes with equal gravity from all sides.
That is why you have the distance from the center of the planet to the surface of the area. The reason is that the gravitational force is applied equally to all sides
Let’s have an example in the same cube shape. Corners are now far more away than the sides from the center. This is not possible if a gravitational force is close to one side and one area is far away from one area.
Another important thing is that the planets are not entirely rounded. The slightest part in the middle is the reason that the cylinders rotate around them very rapidly, because the centrifugal force is slightly out of the axis due to the equatorial bulge. For instance, our Earth is also the fastest in the world, due to the centrifugal force of 42.72 kilometers of the equatorial bulge.