why centre of mass is concentrared in its centre

In the case of a single rigid body, the center of mass is fixed in relation to the body, and if the body has uniform density, it will be located at the centroid. The center of mass may be located outside the physical body, as is the case for hollow or open-shaped objects, such as a horse shoe. In the case of a distribution of separate bodies, such as the planets of the solar system, the center of mass may not correspond to the position of any individual member of the system.