Objects with nonspherical shapes tend to rotate around their center of mass due to the principle of rotational equilibrium. This principle states that an object will be in rotational equilibrium when the sum of the torques acting on it is zero. Torque is a measure of the tendency of a force to rotate an object around an axis.
When an object is not perfectly symmetrical or has a nonspherical shape, its mass distribution is unevenly distributed around its center. As a result, when an external force is applied to the object, different parts of the object experience different magnitudes and directions of force.
If the object starts rotating around an axis other than its center of mass, the uneven distribution of mass will cause a torque to be produced. This torque will tend to rotate the object back towards its center of mass, where the net torque is zero. The object will continue to rotate and oscillate until it reaches a stable position with its center of mass as the axis of rotation.
This principle can be observed in various everyday examples. For instance, if you spin a irregularly shaped object like a book or a phone on a flat surface, it will eventually come to rest with its center of mass as the axis of rotation. The same principle applies to objects in space, where the uneven distribution of mass causes celestial bodies like asteroids or comets to rotate around their centers of mass.