How often in your game does a warrior put a swing into a foe, that an adjacent warrior just finished off, because they'd already committed to attacking that foe?
Rarely if ever, I'd bet - if warrior A finishes off Orc #6 on init 15 then warrior B is sure as shootin' gonna turn her attention to Orc #4 just behind it.
But if those two warriors could both somehow be on init 15 they'd probably both attack Orc #6, perhaps leading to overkilling it but leaving Orc #4 unopposed for the moment. I see this all the time - multiple attackers swinging simultaneously at the same target and all hitting, with each one doing more than enough damage on its own to kill it.
Strictly non-simultaneous turn-based resolution allows far more precise tactical choices than would ever be possible in reality.
Why should the second counterspell always be faster than the first, though?