Because it's fun, and some people want to have fun when they ride. I don't do it myself but if people want to do it go right ahead, worst that comes out of it is that you bother a few people. Bouncing off the rev limiter a few seconds once in a while does no damage to a motor. Some people just love to hear their toy scream and I think there's nothing wrong with it. Granted there's a time and place for it as well