Because we want to ascribe it to something that we have no control over, not our fault, or beyond that which is us. We use it to describe those enemies of our culture and primarily our definition of good. The enemy is always evil, as we are the evil to our enemies. Vilification is necessary in describing those who are different. It is easier to hate an evil, as in the phrase, "evil doers." than it is to intelligently articulate a position. Evil is a rallying point for patriotism and religious zealotry. People are easier to kill when they are defined as evil. They become less human.
Evil is a human invention that incites fear and defines a good, which is invariably, us or a definition of a god. My god is good, your god is evil. It is the difference between us and them. Who cannot be against an evil?
Evil is uniquely human, as is a good. We invented both.