Why is it so hard to convince people of the value of cooperation?

In the classic economic model, the perfect outcome for the rapid growth of a platform is to try to create a network effect. Typically, the greater the number of buyers, sellers, or users, the greater the network effect. You and your company want to profit, and so you pay/contribute/donate something hoping to get something back of greater value. Software and the network effect. If this is so obvious, why is it so often difficult to convince the other guy of the value of cooperation?