It's because of the skill rating of the individual players. Stacks have 0 effect on skill. A 6 man pre-made team can all be sub 0.8s and the other team all solos having 2kd+... You're not losing because they are playing together. You're losing because Bungie forces wins and losses, so bad players and 1kds keep buying silver. If they didn't force wins and losses, they would quit the game because they would lose every game. Bungie proved Fireteam based matchmaking doesn't work after losing 25 games straight on stream. It proves that 6 man fireteams mean nothing if everyone in that fireteam is awful at the game.