beacuse you think you look so good and hot in your little spandex shorts. You get to the track and do some cool-looking stretches, and get into position. You glance down at your slim, muscular legs and think, "I can't wait till the world sees me go." The gun goes off and you run your heart out and win the race.
Then...they do the playback in slow motion. And while watching it, you realize your face is bouncing all over like it's trying to jump right off your head. In fact, though you don't look fat while standing still, on camera while running you see that you have plenty of extra skin on your face. And you wish that the whole world hadn't just seen your cheeks jiggling, because it looks horrible. Even though you have your victory to console you, you will never feel the same about running fast again after seeing yourself in slow motion.