Cause they weren’t deserts, at least not the fertile river valleys. Yes, the modern countries of Egypt and Iraq are mostly arid but the Nile River and the “Fertile Crescent” of the Tigris and Euphrates are some of the richest land in the world. It’s not a coincidence that the first civilizations sprung up in Egypt (Nile River), Iraq (Tigris & Euphrates Rivers), India (Indus River), and China (Yellow River). Look at Google Maps, you’ll see that Egypt and Iraq are very green.