Celestial bodies such as planets, moons, and stars tend to be spherical due to a process called hydrostatic equilibrium. This occurs when the gravitational forces acting on an object are balanced by the internal pressure of the object's material. When a celestial body reaches a certain size, its own gravity causes it to pull material inward, compressing it until the internal pressure balances the gravitational forces.
This process leads to a spherical shape because a sphere is the most efficient way to distribute mass evenly around a center of gravity. Gravity acts equally in all directions towards the center, causing the material to naturally arrange itself into a spherical shape.
Asteroids and comets, on the other hand, are generally smaller objects. They do not have enough mass or sufficient internal pressure to achieve hydrostatic equilibrium, so they do not naturally form into spheres. Instead, they can have irregular shapes, ranging from boulders to elongated or irregular shapes, depending on their composition, formation processes, and history. These smaller bodies often experience impacts and collisions that can alter their shape and cause them to have irregular or fragmented appearances.