Because people will sell stuff for real world money, so it wouldn't be a trade system at all. It would be a system in which people who spend ten hours a day in the game sell stuff to squeakers who play for maybe 30 mins a day using their parents money. Don't believe me? People used to sell Level 50s in halo 3 for real money. people who couldn't get their themselves would buy those accounts to "look cool". A trade system would most certainly be game breaking as well, completely negating the point of rng, Why do the strikes and raids if you can just trade for all the gear you want? Its a terrible idea.