Fleeing enemies would be realistic, yes, but since they would just get hunted down anyway, it wouldnt make much of a difference.
Besides realism, in reallife enemies flee once they realize they stand no chance, not when they're already wounded.
If you're wounded, you're basically done. You can no longer flee anyway, due to blood loss etc.
In BG3, maybe fleeing should make you lose charisma until long rest or something.
Err ...
So Paladin, Sorcerer, Warlock and Bard would get a huge debuff, and everyone else will go "wtf lol whatever".
