I seem to remember reading somewhere that low walls like this weren't really about defense, they were about funneling travelers (with carts) to the tax collectors.
The structure of traveling through that is that you follow the silk road between a bunch of major cities built around oases. It's not necessary to divert anyone with walls - the fact that they need to drink, and that they won't be willing to wander off into the desert, get lost, and die, is sufficient. You don't move people to the tax collectors. You move the tax collectors to them.