otherwise no one would afford to live in such city.
You are right, if you apply European/American demographics.
But you have to realize that the vast majority of Dubai's population is not local. They came from countries with much lower income levels. (India, Pakistan, Phillipines, etc.) So even if the average income minus expenses equation results in a much lower amount than in the West, people will stay on. Because they can't go to the west and they think that they are still better off in Dubai than they would be in the countries they came from.