It is a matter of pragmatism. Even if I myself consider my perspective on good and evil to be objective, it is a given that each of my neighbors will have their own seemingly-objective sense of good and bad that differs from my own. We are then at an impasse. Do I attempt to kill all of my neighbors in order to rid the world of what I perceive to be evil? Or do I perhaps make peace with an imperfect world in which bad things happen in other countries that are not my jurisdiction to worry about? Apparently you subscribe to the "kill all your neighbors" camp, that your objective brand of morality must be enforced on the entire world by means of military might. World conquest, however, is an utterly irrational thing to attempt, and will only lead to death and destruction, not an idealistic world that conforms to your sense of morality.