That's not quite right. Always defecting is game theoretically optimal. However, under realistic conditions, where many agents are predisposed to cooperate but

That's not quite right. Always defecting is game theoretically optimal. However, under realistic conditions, where many agents are predisposed to cooperate but some do not that's where a tit for tat strategy can work incredibly well. That people are predisposed to cooperate is an assumption you have to make, it cannot be derived from the rules of the game itself.