"It's so unlikely we shouldn't worry about it happening again" is a reasonable attitude toward a hurricane or tornado. Weather doesn't have a will. It's not seeking weaknesses. It doesn't want to hurt us. War is different.
America has enemies, people who want to run the world according to specific lines that we interfere with. Right now the Islamofascists are the most prominent ones as they try to establish a Caliphate. They have a seed of that in the Islamic State straddling Iraq and Syria. Osama bin Laden wanted to establish one. The 9/11 attacks could have given him the stature to do it if the US hadn't struck back so hard.
The 9/11 attackers would have caused ten times as many deaths if they could. They would have if the builders of the WTC hadn't done their work so well or if the workers had panicked instead of evacuating in orderly fashion and helping each other. There have been other Islamic terror attacks on the US, the LAX shooter, the Times Square bomber, the Fort Hood shooter. The casualties there were limited by the competence of the attackers, not their malice. They would have killed many more if they could.
The Islamic State's Caliph Ibrahim is too busy to sponsor attacks abroad yet. If he gets some breathing space he'll need to. The Caliphate isn't inherited. It's a "Mandate of Heaven" that belongs to the ruler who can beat up everyone else around. Ibrahim has signed up for perpetual war and attacking the Great Satan will be required of him . . . or he'll be considered a decadent pretender and wind up on a meat hook.
We are at war. We need to take and hold the initiative in the war, or we let the enemy decide the time and place of the battles. In this war this means the death of civilians on peaceful sunny mornings.