Game theory bases that we are all self-interested and that also adds a level of safety in some scenarios. For instance you don't want to tank your team, you want YOUR team to win so you do more because of that self-interest. Cooperation can be part of self-interest in that aspect. Even self interest has a level of moral norms and respect for civil rights because you want that same level of protection.

What happens to niceness? It gets chewed up and eaten. I am a very cooperative person but through my life I have realized niceness/loyalty come at a cost but are hard not to do. If you aren't self-interested in yourself, noone will be.