1. All the three buildings collapsed at free fall speed. This means there is no resistance from the bottom supporting structures.
2. The fire was oxygen starved so it did not achieve its full temperature. People were standing in those floors where the planes hit. At these temperatures the steel doesn't weaken much.
3. There is not much fire in many of the floors below the impact zone, still they offered zero resistance to the falling pieces from above (almost powder)
4. All the upper floors became small pieces (most of it powder) are ejected sideways and cannot offer enough force to crush the remaining floors in such a short time.
5. No steel structure ever collapsed due to office fire prior to or post 911 because it is impossible. Only controlled demolition can do that.
5. If office fires can bring sky scrapers completely and orderly like we witnessed on 911, the demolition companies will go out of business.