Because the way gravity acts is to pull towards the centre of a planet. On a sphere, all the points on a surface are an equal distance away from the centre. Any objects that rise significantly high will have a bigger gravitational pull acting upon them, which will pull them down. So over time gravity will pull the planet into the most efficent shape, a sphere, where ever point on the surface will have an equal gravitational pull acting upon it.
If there were a big object on a planet, gravity would pull it down, which is why you don't find 50 mile high mountains on planets. Equally, if the planet were shaped like a cube, the corners of the cube would be further away from the centre than the rest of the cube, meaning there would be a bigger gravitational force pulling here, so would eventually they would be pulled down, resulting over time, in the planet having a spherical shape. You see?
If there were a big object on a planet, gravity would pull it down, which is why you don't find 50 mile high mountains on planets. Equally, if the planet were shaped like a cube, the corners of the cube would be further away from the centre than the rest of the cube, meaning there would be a bigger gravitational force pulling here, so would eventually they would be pulled down, resulting over time, in the planet having a spherical shape. You see?
