I suppose it might be connected with what in psychology is called dual processing. That human beings have two ways of thinking. There is system 1, which is unconscious, irrational, and automatic. It is evolutionarily old and a sort of non-verbal thinking that animals have as well. And there is the evolutionary recent system 2, which is analytic, rational, and using logical standards. As Manchegan wrote, working and living together best serves our natural interests. Cooperation is the more logical and beneficial decision in the long term - at least that's what I understand from Game Theory - so the choice of the irrational and non-cooperative behaviour might be triggered by system 1.

Well... I'm not convinced that my speculations are right. I have neither a deep understanding of psychology nor I'm expert on Game Theory. I just think it would be interesting to investigate if there is a connection.