If you notice all of the great ancient ruins are located in arid or desert regions. If a drought occurs, especially in an agricultural society the people will starve unless they pack it in and relocate.
Many ancient sites were built around the time of the end of the last ice age where regions that are deserts now were once lush with an ample variety of flora and fauna. With glacial retreat, the water became more scarce and rivers, streams, even lakes dried up. The aquifers helped some groups hold out with the cisterns in their fortresses but that resource only held out so long.
Basically in ancient or primitive societies they relied on environmental factors for survival. This is one reason many great civilizations seemed to just vanish...they left for greener pastures elsewhere. This is also why they formulated a highly accurate calendar system. The "Cedars of Lebanon" were once in a thicket of dense, humid and even rainy lands.
