No matter how unlikely an extreme event might be, if/when it does occur it has earth shattering effects due to the complexity of the world. This is why we need to be so vigilant in protecting ourselves against these threats.
I finished with "But September 11th came along and changed the West and Middle East for decades." That is the point. 9/11 was tragic enough, but it triggered an avalanche of death and trillions of dollars spent. These sort of attacks have non-linear effects that cannot be predicted.
You originally stated you are more likely to die falling down the stairs therefore "Bomb attacks are not something you should be concerned by." My response is that terror attacks are like what we see in 9/11. Considering that event resulted in hundreds of millions of death due to the complexities of the world AND the risk distribution and magnitude is completely incomparable to known risks and their distributions, it is nonsense to dismiss the risks as you are more likely to die falling down the stairs.