Because it was designed to take the impact of an airplane. A B-25 bomber hit the 78th and 79th floors of the Empire State Building in 1945 and other incidents happened after that involving airplanes hitting high rises on accident.  Engineering advances were made to make sure that they wouldn't fall.  It gets very foggy in NYC and it was a real concern when building these towers.  The Twin Towers were specifically designed to withstand airplane hits. https://en.wikipedia.org/wiki/Construction_of_the_World_Trade_Center#cite_note-110 On July 28, 1945, an Army Air Corps B-25 bomber crashed into the 79th floor of the Empire State Building. Fourteen people died and $1 million damage was done to the building. The structure of the building was not severely damaged.  The B-25 is a medium 2 engine bomber.  It is 53 feet long with a 67 feet wingspan.