But it would seem far more feasible to build an organization like that to oversee one unified police force than to have n tiny organizations to oversee n small

But it would seem far more feasible to build an organization like that to oversee one unified police force than to have n tiny organizations to oversee n small local police forces. If you'd try to mix levels (one central org cross-checking n small forces) everybody involved would inevitably confuse their role with a central coordination hierarchy and the original intent would get lost.