A world court doesn't require a military in a highly interdependent world. Economic sanctions are sufficient. A city state is going to require food and goods

A world court doesn't require a military in a highly interdependent world. Economic sanctions are sufficient. A city state is going to require food and goods from other city states. If you go against the ruling of the world court, other city states refuse to provide those goods, and you are going to fall into line fairly quickly. Another benefit of smaller independent city states is that it is easier to check their aggression. If raising a large army requires broad cooperation among city states, that greatly reduces the likelihood that such a force will come into being. This is because of the difficulty in coordination and the fact that multiple city states are unlikely to share the exact same interests, reducing their impetus to unite.