The problem is that they are not always able to keep some kind of perspective on this obsession unlike most adults. So they may either start playing in a very defensive way, or just always give the ball to the big guy on their team. In general this means that they fail to explore and express themselves and develop skills. In the worst case they just drop out.
So the aim of de-emphasising the winning at a young age is so that you develop winners in the long run. New Zealand, the world champions in rugby, do this. The end product is a ruthless winning machine.
Children are not small adults, mentally or physically and most coaches have to be taught this explicitly (including me)