It's easy to be cooperative in a mutually beneficial activity. However, most competitive activities such as games and sports require some cooperation in the form of following the rules. Society has to instill this value in us. I remember as a child playing a board game or a card game and feeling an urge to cheat and being societally pressured into following the rules. It was considered a part of growing up. This is because we are little sociopaths when we are toddlers. We simply don't give a shit about anything other than getting our way and don't think the rules should apply to us. Competitive activities build the strength of character to resist the temptation to violate social contracts for personal gain and they teach us to keep our emotions in check. Even the most competitive sports, such as wrestling, boxing, and mixed martial arts require rules and cooperation. You can't bring a baseball bat, you can't simply cheap shot your opponent before the match starts. The point is that it's hard to follow the rules in competitive activities and we have to be taught to do so because it goes contrary to our selfish nature, whereas cooperative activities don't typically tempt us and don't test our character.