Because there's a need to be random...
Marriage Traditions of Foreign Countries
It is a tradition for married couples in foreign countries, upon driving to their honeymoons, to tie small children to the back of their bumpers instead of cans. "The cans make a more rickety noise, which can be hard on the ears," claimed a newly wed groom. Upon reaching an overpass, the married couple "reels in" the children, wraps them in white garments, and drops them on unsuspecting passing cars below. A small piece of paper is tied to the child reading: "I am from the stork. DO NOT ask questions."
-Kinoko Otoko