On the plus side, the land is generally flat, and not all that useful for any other purpose (without a lot of irrigation), and is generally considered "wasteland". So instead of locating millions of people on land that's better used for agriculture, or left to wilderness (like major forests), putting people in a desert wasteland may seem like a good idea.

On the other hand, it's extremely hot there, and not really an enjoyable place to live unless you like being inside most of the year. On top of that, all the asphalt and concrete causes a "heat island" effect, making the city even hotter than the surrounding desert, and unlike the open desert, it never cools off at night (all that concrete and asphalt constitutes thermal mass). And since people don't like living in 100F+ conditions continuously, a lot of energy is used running air conditioners to try to make the place habitable.