They were sophisticated enought to synchronize the theft of four airlines, time their collisions with targets (except one) and fly them into buildings. They were quite sophisticated enough to assess the structural weaknesses of the WTC towers. I would hazard a guess that bomb-proofing was not part of the original specification for those towers, nor the budget. They were strong enough to stand comfortably for years in all kinds of whether. Maybe noone thought they would be military targets.

A