Skyscrapers are unique buildings which involve a very complicated building process. Leaving aside the massive difficulty associated with designing the mechanics of the building to ensure that it stays standing, the actual build also needs to be planned very carefully. The logistics need to be very exact so that the build work is done as efficiently as possible. If you look at the way the new world trade centre was constructed in the USA, when every floor had the structure completed, the workers installing plumbing and electricity came up immediately, so that there was no lag time involved in the operation.
While that was impressive work, there are still complications, which you can see clearly in the way that the Burj Khalifa was designed and constructed. One of the biggest difficulties in constructing such a tall building was finding a way to deliver the steel support beams to the top. Typically they would be lifted by a crane, but even the tallest cranes were incapable of reaching such heights, so the building was constructed with multiple platforms, which when completed would serve as a platform for the development of higher levels.
