It's as Visscher said earlier, it's gameplay over realism. I would get very angry and frustrated if my army started to flee even though I know if they would have stayed I would have won. I don't want to lose because of my soldiers stupidity. I want to lose because of my own stupidity if anything, at least then I know that I can improve, and I don't hate the game as much then.
Picture made by Aametherar the orc (Elves are the best!)