What did ancient mesopotamia offer early people that encouraged them to settle and stay

Ancient Mesopotamia had excellent condition for complex human societies to develop. This area had two large rivers, thus there was water. The river valleys had lot of deep and fertile soil, excellent for agriculture. The climate was constantly warm which was excellent for the crops. These conditions were mostly good for the agriculture, and since the agriculture was the basis for the production of food, it enabled the people to solve their basic problem, have spare time, and engage into other activities.