What Kevin Dick said.

The benefit to each player from mutual cooperation in a majority of the rounds is much more than the benefit from mutual defection in all rounds. Therefore it makes sense for both players to invest at the beginning, and cooperate, in order to establish each other's trustworthiness.

Tit-for-tat seems like it might be a good strategy in the very early rounds, but as the game goes on, the best reaction to defection might become two defections in response, and in the last rounds, when the other party defects, the best response might be all defections until the end.