But in the end you have several tons of concrete covering tens of square metres of ground, to allow one or two square metres of cereal grains to grow. That is not particularly efficient use of time or resources.
Furthermore, there are several techniques to help trap moisture that does not require a concrete lid. I'm in the Phnoenix desert, and I use the trimmings from my zucchinni plant as green mulch. Along with the palm fronds that my neighbors love to trim, they create very effective moisture traps. I need to trim my zucchinnis anyways for their optimal health.
I could also plant things in a way to take advantage of the vertical spaces as well. Okra, for example, thrives in the low desert, and provides dense shade that already traps moisture. If I add something that likes shade, and stays closer to the ground, that forms a kind of living mulch that does not require a lot of ongoing maintenance. And if I plant pole beans to take advantage of the sturdy okra stalks, they fix nitrogen and help keep the soil fertile.
I didn't come up with these ideas. They may not be widely known, but people have come up with these design patterns and used them well.