It's all about physics and structural engineering.The towers were a marvel of engineering.Nonetheless if you take out the right beams at the right spots any engineering marvel will collapse.Think about the enormous amount of heat created by the burning fuel; steel weakens when heated to extreme temperatures. Plus a lot of force was transferred on impact.767s weight about 450,000 lbs including 24,140 gal of burning fuel.If the fuel wouldn't have burned then MAYBE the towers could still be up.