Most people survived them though so there wasn't a huge reason to do anything particularly drastic, using planes as a suicide missile was basically entirely new. Even on the day of there was speculation about why the planes 'messed up' and flew into the WTC buildings which shows just how unusual it was. Most hijackings were essentially mass kidnappings for ransom to get money, release prisoners or get promises of policy changes and ended with a negotiation or storming the plane.