The point is that you’re not face to face, but seconds away from engaging. Terrain might slow units down before they reach each other, but not by that much. Everything about the map suggests this, through the size of the map, the scale of the units and objects, the amount of AP you get per turn, the lethality, how far you can move, etc. To clarify, It does not take minutes to swing a sword twice and for the other to dodge and counter-attack, but seconds. Same goes for running past a tree or readying your shield.
You’d want to rush when time becomes something that can result in your position on the world becoming worse, which is what would happen if time moves at such a pace that reinforcements become viable. This is because for reinforcements from either side to a viable factor, time would have to move at a rapid pace as otherwise they’d never show up. Time moving that fast results in the player always getting punished for taking their time, rather than their approach being dictated by the circumstances of the battle itself, because time passing is bad for the player as it means losing resources. Furthermore, you’d get a disconnect between what you see happening in combat and the amount of time that you’re told is actually passing.
Also, having fatigue cost increase by a percentage per turn would also quickly result in combat being completely crippled. Having low-fatigue characters would be impossible, undead would become invincible and the only viable way of fighting would be killing the enemy as fast as possible.
I just don’t see adding passage of time being worth the effort or actually improving the game. It’s pretty much fine the way it is now.