The kids who had played together, worked better together.
The kids who hadn’t played together spent most of their time defending their pile of junk, and trying to steal or grab junk from the other piles. Even though the materials were purposefully selected to be of the no-apparent-appeal-to-anyone junk variety, they spent more time fighting over the materials than in building with them.
The kids who played together eventually built a single city. They started out, dividing themselves into groups around each junk pile, building streets and houses and apartments and playgrounds, and eventually built roadways to collect their cities together into one metropolis.