Why did Rome unite into a single state, but Greece did not succeed?

The Roman Empire was able to become a powerful state with a centralized power, unlike Greece, since it formed around one city – Rome. Greece was divided into dozens of cities of states that endlessly fought and competed with each other. These cities differed in size, wealth, and method of government. This weakened them in the end. And the Romans all the time seized new territories and annexed them to the main lands, carried out reforms in government and in the organization of troops.