They see our reactions to the first WTC, embassy bombings, Somalia, USS Cole and they think that by applying the pressure, we will wilt and run away. They see what is happening in our country and how a long, messy war lessens our resolve. They are using that to their advantage. The American public cannot stomach this type of war for too long. That is exactly what they want and they are succeeding.
That is why they continue the attacks. They do not see it as "well, if we stop, then the U.S. will leave."