Player-to-player trades are in the game. It's just very rare that two teams will have players that they are both willing to lose, fill each others needs, *and* are perceived as equivalent in value. This is the same reason its rare to see player-to-player trades in real life.
As an example, I tried to see what I could get for Jason Taylor (as Miami), and various teams offered to trade Bernard Pollard, Ryan Tucker, Ken Dorsey, Marlon McCree, Dexter Jackson, and Willie Colon. All of these offers came with picks included.