It's a marketing tactic to sell more copies and to emphasize the social aspect of the game, I believe it was said that the idea of players being able to interact with eachother and help each other was what helped give the initial concept the greenlight
maybe they originally wanted people to buy link cabels but when link cables were phased out it had just become tradution at that point?
maybe they want peoples playthroughs to be different from eachother?
it's probably because they want you to pay more...