My first thought is that that kind of location also makes invading harder. For thousands of years, armies had to acquire most of their provisions on the road because logistics are hard when you're marching thousands of people on foot, and "acquire" usually meant "steal" even on home territory.

Being invaded fucking sucks, so it's quite possible that a lot of people were willing to trade personal liberty for security.

I thought it was because land around the river is more fertile than in the rest of the desert

It's really easy to think of examples of that, wow. I don't fully buy it but i can see how it could be a factor in some civs.