Because you most likely will wait until the other person is right behind you, slowing them down.
Even a slight tap on the brakes send ripple effects through highway traffic. Im sure youve experience a 15mile traffic jam that suddenly opens up, and there was absolutely nothing there. One person brakes, so the people behind him and so on and so forth.
And also, this might come as a shock, but youre not perfect. You probably dont look at your review mirror every 7 seconds, because youre on cruise control, the road is straight, and your favorite song is on. The guy who just came up behind you? Maybe hes been there for 30 seconds, pissed off at your incompetence.