Because of gravity. Actually, rock is not solid when it is subjected to very high pressure and temperature. For bodies larger than about a few hundred miles in radius ( like the asteroid Ceres for example) rock starts to become plastic and begins to flow. The larger the body, the more plastic its interior becomes. Now gravity, according to Newton's law of gravity, is a force which depends only on the distance between two bodies. It is said to be a 'spherically-symmetric force' for that reason. This means that for chunks of rock at the same distance from the center of a planet, the forces will be equal. The consequence that this has for the shape of a planet is that the rock will flow until, at a given distance from the planet's center, it feels the same forces at a given distance from the center. This shape will, therefore, be that of a sphere.
Also for larger bodies, the accumulation of matter into the assembled planet is a process that carried energy into the planet. The chunks of rock collided with the planet and the energy of the collisions was converted into heat. Once a planet gets large enough, it becomes harder and harder for it to dissipate this heat into space faster than it accumulates it, so the interior of the planet heats up. Earth's interior is as hot as the surface of the Sun...5000 K, and for Jupiter the temperature is some 56,000 K. So, this contributes to the rock in the planet's interior being very plastic, molten and deformable. There are no cubical planets because this shape cannot be supported by fluidized, molten rock under the force of gravity. Planets can become slightly flattened spheres due to their rotation speeds, but once a planet stops rotating, its shape will very quickly return to spherical.