The rest of the army has been destroyed, and the only thing they can do is to kill the swarms of enemy soldiers. The only way to do that is to wait around and watch them die. They're not going to be able to do this if they just sit around and wait for the battle to end. They can't even imagine what it's like to be a rear-guard soldier, so they're going to have to do whatever they can to get out of the way.