You think that airport security has some kind of mythical aura around it that keeps these things from happening? Hardly! Your basic argument seems to be "Such a hijacking could never happen, because it sounds so ridiculous when written down." Well, my friend, the world is fvcking hilarious. Illogical, even stupid, things happen every day. And the easiest group in the world to manipulate is a group of mixed strangers, such as on an airplane. Sure, there's no way that a terrorist armed with a box cutter could fend off an entire plane full of passengers. Do you want to be the first person to stand up and fight him? No? Someone else will do it, surely. After all, I haven't had any training. Maybe they'll be an air marshall...it's their job, after all. As soon as they land the plane and make demands, just like every other hijacking, Captain America himself will command a squadron of Rangers to free us all. Happens every time! Even if the terrorists kill a couple passengers, surely it won't be me...what would be the odds of that happening?
The reason why it worked, was because it hadn't been done before. US security is a low-level job filled with unimaginative sorts. They take their job seriously, but they only look for the threats they have seen before.