I've thought about it over the years as well. As Mary Anne says, the strikes were apparently high enough the the impacts were absorbed, simply by the height of the buildings. A lower strike, near street-level, might have undermined one side of the structure enough for it to topple. Another point to consider is the speed of the impact and the mass of the planes. They were carried well within the buildings, so the main fires that weakened the central structure was centrally located.

Actually, a good demo-man on the ground could have made a bigger mess of the place with an off-balanced set of explosions, but it would have required a lot of unobserved time in several support locations, adding up to hours of work, just to set it up.

Dan