The building was not a solid slab of concrete, it consisted of open plan floors with windows.
True -- Though there are countless pics videos of planes hitting each other & or other objects
less structurally dense / strong as concrete & steel & the planes disintegrate into pieces
Not remaining intact & exiting nose cone intact through the others side
They are aerodynamically designed to cut through air - not designed for with standing solid frontal impacts
Heck have you seen the damage birds can cause to planes
And No i'm not purporting to any conspiracy.