Why are river ambushes so much like bridge battles?

[White Knight2]

So many times when my army ambushed an enemy near river crossing in the ensuing battle the armies were placed on the opposite banks like a standard bridge battle formation. Well in that case the whole point of the ambush is lost. You have to first get your army across the river to attack, by which time the enemy is smugly waiting for you to come over. At first my perception was this only a one time case but its happened on almost every single occasion. WHY IS IT LIKE THAT!!!!

[Crymson]

So many times when my army ambushed an enemy near river crossing in the ensuing battle the armies were placed on the opposite banks like a standard bridge battle formation. Well in that case the whole point of the ambush is lost. You have to first get your army across the river to attack, by which time the enemy is smugly waiting for you to come over. At first my perception was this only a one time case but its happened on almost every single occasion. WHY IS IT LIKE THAT!!!!

It's hardcoded. Ask Creative Assembly.