I suspect it is a size issue. If you’ve been to the 102nd floor of the empire state building, there is not much room, so the 87th to 101 is too cramped to be rentable.
With regard to the Burj Dubai specifically, the top few dozen “floors” (I forget the exact number) are just landings on the steel access stairway up the spire to the aircraft warning lights on top.
This cross section should make it a bit clearer.
The top of the concrete structure is level 155. Then there is a steel spire on top of that. The highest habitable floor is in the lower, wider part of the spire. Above that it’s just a narrow structure with a staircase going up it.