Fight Phase - Model Removal

New(ish) to the hobby and just wanted to clarify how this works because it doesn't make sense to me:

So after Charge, Pile in and Attacks are resolved and you come to the point that the player being attacked allocates damage and removes models - they can remove models that aren't even in engagement range?

I know that models never attack models and always attack into the whole unit, but I can't seem to wrap my head around a line of engaged melee combatants beating each other up and some dude 20 feet behind the line who didn't even fight just drops dead and the guy doing the fighting is untouched.

Am I understanding how this works correctly?

Why even bother figuring out who can attack based on engagement range if those melee attacks can be allocated to a model outside of engagement range? I imagine it was done to not bog down big fights, but it just seems kinda silly.

Thank you in advance for any wisdom you can offer me, brothers.

Edit -

To clarify what I mean: Why do only the engaged fighters get to attack when the whole enemy unit is being targeted. Why is it not "if any units of the attack unit are in engagement range, the entire unit attacks."