Why hasn't anyting happened?
because al qaeda is a radical political group (i.e. terrorists), not an army. people forget that.
the attacks of 9/11 were chosen not due to body count, but due to backlash. for an attack of that magnitude, 3000 people dead really isn't that much. a handful of suicide bombers at the macy's day parade could have create far more chaos.
attacking the pentagon, the white house (shanksville, supposedly), and wtc were symbolic attacks. and that's the point: symbolism. terrorists do what they do to send a message, they do it to terrify. currently, the USA is showing that we're still terrified.
if a ragtag team of idiots in miami years later can get the whole country freaking out, if one attack can launch a giant-scale war that ends up swelling their numbers more than eliminating them, if one act can get the country to start giving away all of those freedoms and liberties they supposedly hate, why bother with another attack?
had we gone into afghanistan, done the job there, killed osama, then left and rolled along like nothing happened, we would have sent a far more powerful message. a message that says "you can't scare us. you attack our citizens, we eliminate you and get home before dinner."