If you start looking at the floor plates for each floor in these super tall buildings, the first 50% of the lower floors have very little usable (rentable) space, since the elevator shafts use up so much of the floor space. Many elevators don't go all the way up the building, so the upper floors generally have more square meters of usable space. And the upper floors generally rent for more money per square meter due to views. Overall, developers are hoping that the prestige factor of the building nets them more money in rents to offset the additional costs in building such a building.