The reason is that it IS a teamplay game, but there just isn't any objective... There's no reason to play as a team. If robbers had to rob a bank, then yes, people would work together to stop them, and the robbers would work together to rob it.
Without an objective, there's no reason to help teammates.
EDIT: another way of thinking of this:
In this game mode, you cannot lose, or win. There isn't any reason to try.