Selfish apes have a harder time finding mates, and getting help from other apes. Either way, they tend to have fewer baby selfish apes. Not only that, tribes of mostly selfish apes will have trouble working together for a common goal, so they'll be less successful as a tribe.

On the other hand, every tribe needs its share of troublemakers, rabble-rousers and risk takers, natural leaders and street fighters, i.e., selfish apes.

So the most successful troops will, after a time, reach a balance, have just the right mix of rugged individualists and team players, and they will be more successful competing against other troops. In all human social groups, there is this tension between working together (there is no "I" in "team") and individual excellence and competitiveness and aggression (try harder, never quit, never settle for second best, everybody loves a winner, develop your leadership potential).

Look at competitive social groups, such as gangs, military units, athletic teams, corporate offices. There is great emphasis on team effort, but there are struggles and rewards for the alpha male, too.

Nobody has gone to the trouble to set it up like this, its just the way its worked out. That's what apes do.