There are a few periods in time were city states dominated a landscape, in specific I'm thinking of classical Greek city-states and renaissance Italy city-states.

In a world of despots, god-kings, military rulers, and larger nations (especially around the time of Italian city states, and compared to Persia for Greek city-states), what fostered the existence of these smaller, more autonomous city states?

Was it some sort of power vacuum where no one chose to conquer the smaller states? Did they have grassroots nationalism that kept them autonomous? Did no one want to combine into a larger nation? I really have no idea and am interested why these pockets of time existed in these areas. (Italy and Greece spawned my question, but it applies to Fertile Crescent city states and south-east Asian city-states if applicable). Thank you!