The statistical likelihood of something happening isn't super relevant to how we as human beings deal with things. Fear is a huge motivating factor. While we do not live in a perfect world, police, by design, decrease fear while terrorists increase it. That's why we commit money and resources.

A nice analogy is the difference between flying and driving. I know many people that have a fear of flying and refuse to get on a plane, but are perfectly comfortable behind the wheel even though it's statistically much less safe. How people feel about things is important.