It's harder than it seems. Architects and engineers have learned a lot of valuable lessons from what happened to the WTC. Those towers were designed to withstand an impact by a Boeing 707 sized airliner. The 767s that hit them were larger and heavily loaded with fuel.

The towers withstood the impact and remained erect long enough for thousands of people to get out, a pretty remarkable achivement when you think about it. Tragically, the fire escapes, elevators, and water lines were cut at the points of impact. Could they do a better job of designing those critical services to withstand impact? Perhaps, but the planes keep getting bigger, like the new Airbus A-380 currently under development. If the architects design a skyscraper to withstand an A-380, what happens when the next generation of airliner comes along?