So why don't they cooperate? - because it is to risky. Cooperating is for suckers.
Hobbes (1588-1679) founder of modern contractarianism. Leviathan, imagen a situation without a government or authority. He calls this the state of nature. The worst place you could ever be.
War of all against all. The lord of the Flies. Somalia, Sudan, Afghanistan etc.
The Hobbesian state is a prisoners dilemma. Everybody is seeking to maximize their self-interest.
There is a escape from the state of nature: we need two things: beneficial rules that require cooperation and punish betrayal, and an enforcer who ensures that there rules are obeyed.