So I have this planet in an alternate dimension, which is Earth-like but slightly larger, much more volatile, and orbits an orange star. This planet's name is Geios (uninspired I know) and has three major continents, two ice caps, and two major islands along with some smaller island chains. So large landmasses but also lots of ocean. The total population is about 2 billion. They have advanced technology, however air travel is rendered all but impossible due to the very volatile atmosphere, and fossil fuels are very rare, so automobiles are not common either. The climate is harsh, with large deserts and mountains being common on all three continents, and as a result the primary form of civilization is a group of massive independent city states and several more nomadic tribes.
So why would independent city states be preferable to larger but obviously more spread out nations? I had ideas that putting everyone together in one place made it easier to protect them from harsh weather and even harsher raiders, but what other advantages are there?