“Why” to me typically implies purpose but I suspect you’re asking a social science question which is actually a “how” question. If I were to answer “why” I’d say “because the universe sucks.”
But if you’re actually asking something like “How does selfishness confer advantage in groups?” My response is that a cooperative group is defined by denial or at least delay of fulfillment of individual need in favor of group needs.
A good example is planting seeds instead of eating them. If I’m given the town’s supply of grain to plant but instead I make bread for myself I thrive and everyone else starves.
It seems to be an intrinsic facet of the universe that exploitative behavior is inherently profitable in certain contexts.
(Which is why capitalism ultimately will need leg braces: http://underlore.com/one-possible-solution/ )
Think about 3 kids growing up alone in the woods. One kid is selfish, one kid is apathetic, one kid is generous.
As they grow the generous kid will give away more of his food, the selfish kid will steal/take, and the apathetic kid will have his food stolen but also get free food from the generous kid. The selfish kid being better fed will get bigger leading to superiority in those contexts while the other two would wither.
The selfish kid dominates within the group as advantage breeds further advantage.
Now imagine a another group of 3 and these kids act as a team. Economy of scale grants these kids an effective food supply (and everything else) of 4 or 5 kids so that each of them is larger than the average of the first group. They fight. The 3 coordinated kids beat the larger leader of the first group and absorb the other two kids.
The sharing culture beat the selfish culture, but only when the cultures had the opportunity to clash.
I’m sure I could have explained that better, but then again that’s what you get for asking me instead of Google š
Because game theory:
http://www.aaronsw.com/weblog/tdk
http://adamcurtisfilms.blogspot.com/
Because selfishness:
http://www.popsci.com/scitech/article/2009-08/evolving-robots-learn-lie-hide-resources-each-other
