Because in that case with Steam, one person could play his games, trade it to his friend for 1 cent, who trades it to his friend for 1 cent, who trades it to his friend for 1 cent, who trades it back to the original owner for 1 cent.
So four people got to play a game, use all of its online features and utilize the full Steam functionality, and three of those people didn't pay a cent to Valve or the game's publisher or developer for using their service. It would actually be cheaper for Valve if they pirated it because they wouldn't have to pay server costs for freeloaders.
I'm not gonna say that a consumer just shouldn't have the right to do that, since they can with physical copies already, but it absolutely would affect Valve's business model. To me it doesn't seem fair to rip the rug out from under Valve's feet and upturn their highly profitable business model that people 100% have the choice of whether or not they buy from.