Maybe you're just bad at the game. Maybe you think the game is something that it isn't. Maybe you believe the original hype that no one can beat more than one person at a time. I killed all the Cumans and bandits in the camp without saving. If you can't do that, then you're either bad, lack imagination, or take what is told at you at face value. In the alpha, people were complaining they couldn't kill two Cumans.
It worked better at Agincourt, but the odds were less impressive than they were at Thermopylae. If the AI can find a goat track to outflank me, then I might be in trouble, except for the fact I can move, and they were there to delay the Persians until reinforcements could arrive, which they did. If I simply stand still and get myself surrounded, then my reign will be short and unsuccessful.