Because that's what it is. It's like saying that we have to rid the world of Communism by killing every person who believes in Communism. We defeated Communism, but that's not how we did it. We defeated them by the force of our culture of freedom, which we propogated through various channels throughout the communist world. Yes, we had our military as insurance and that played a large role, but if the people of the Soviet Union didn't know and admire the culture of freedom of the United States, our military would not have achieved the victory alone. In a similar manner Islam must be defeated, not through the sole use of our military but by a vigorous propogation of our ideas and values into Islamic society. And God willing, those societies will one day by "formerly-Islamic" societies.
While a "vigorous propogation of our ideas and values into Islamic society" is not beyond possibility—"where there's life, there's hope"—it certainly seems unlikely. As we discover more and more what Islam REALLY means to do to us and for us, the question is WILL WE HAVE THE GUTS AND THE FAITH TO TAKE THEM DOWN or will we surrender?