This seems to me to be an issue of how many cities existed in massive river valleys versus not.

Originally 100% of cities were in massive river valleys.

By the late bronze age, other cities existed, but those were mostly within the empires of river valley civilizations.

By the time the Greeks started writing again there were many costal cities, and relatively few new cities in-land, so you got a lot of thalassocracies (Phoenicia, Athens, Carthage), because that's mostly what the new empires could be.