Well for one thing, a game generally lasts up to say, 30 to 60 minutes. Now if you have a bad player, that person will probably wind up dragging the team down, either by not farming well, or even giving the enemy team kills, making them much stronger, and inducing a snowball effect. I'm unsure of surrender times, although in League of Legends, it's about 20 minutes (given that it passes and no one says no out of spite). So 20 minutes of a loss, or a entire hour of slowly winning. It's even worse in Dota 2 because at the time of this writing, there isn't a surrender function implemented in the game yet.