Airports (not airlines) provided security against known threats. The 9/11 hijackers circumvented that bringing in box cutters, not the expected guns and bombs, and by using the planes themselves as weapons, instead of just hijacking the plane and using the hostages as leverage.
TSA is in precisely the same reactive mode: make people take their shoes off after some nut tries to blow up his shoes, etc.
Bruce Schneier: "Only two things have made flying safer: the reinforcement of cockpit doors, and the fact that passengers know now to resist hijackers."
