Various reasons, of course, but a key one is that we're lazy: formal logic needs to be applied to something, but apparently accusing the other guy of fallacies is more fun than the boring old job of taking time to define one's terms. How many arguments have you seen that keep going back and forth because each side is using slightly different meanings or understandings of some concept. Yet almost never does anyone stop to say, "Hey, what exactly do you mean by X? I'm using it in this sense…"