That's because Hollywood is visual.
Imagine a Zombie not as a biological organsim (Ala Resident Evil), but instead as magical energy animating something. Just like animating a statue, so too is a zombie; just a zombie is easier to destroy, as aposed to a statue. So, you could animate a zombie, even if it doesn't have a head. Or an arm. Or is missing a jaw.
Furthermore, on this same vein, you could shoot an arrow through it's eye, and it just hits dead tissue. If it were a Human, and that was a critical hit, and it did enough damage, sure, they dead. However, the zombie isn't phased (Unless the amount of damage killed it).
You chop off a zombie's arm, he keeps on coming. You chop off his leg, he keeps it up. You have to just wear him Down. Zombies that are affected by critical hits would likely be those in Demon Night, and Resident Evil.
Heck, I think it'd be cool to animate just PART of a skeleton. Imagine simply skeletal arms which are animated; put a lot of them in an underwater passage, and you have yourself a defense.
