Why did the earliest civilizations arise in the valleys of great rivers?

Ancient civilizations were agrarian. For the development of cities and the division of labor, it was required to obtain a sufficient amount of food (for artisans, merchants, townspeople, priests, bureaucracy). It was obtained not by hunting and gathering, but thanks to agriculture. It was convenient for them to study where the climate is warm and where there is a large river that overflows and brings fertile silt. For example, the Nile in Egypt, the Tigris and Euphrates in Syria and Iraq, the Indus in Pakistan, the Ganges in India, the Yangtze in the Yellow River. The exceptions were the civilizations of Urartu, Elam and Phenicia. They were less associated with rivers, while the Phoenicians were merchants and sailors.