Aardvarchaeology has a nice article about abandoned treehouses.
These sites and their formation processes reflect children's psychological characteristics. Kids have little sense of order, short memories and strange rationality. They also have no idea that childhood is brief and transient. They will happily fill their treehouses with junk without any thought that they might one day stop coming there.
And when they stop coming, everything is left wherever it dropped the last time they played with it. Link