Oh, I know, but then re-issuing the charge command leads them to pursue the nearest unit. Which in the scale of a large battle, is usually some infantry unit running away, which results in like 6 horses chasing after one fleeing soldier, instead of charging full on into an infantry assembled line formation... or into the archers to prevent them from raining **** on your troops.
Oh an also - constantly getting your troops to stop fighting and follow you, can result in unnecessary troop losses, since their priority is to follow you, and not engage... So really, it just isn't that effective at all.