These are Horror Movies. If one wants to include the soul in the equation, it would be much more horrible to think an innocent person is tethered to these monsters and must uncontrollably take part in the violence, death and cannibalism.
Oh, I can accept that, but if they have no control over what's going on, then that doesn't explain where the apparent memory and higher functions come from in some of the zombies. It seems contradictory to imagine a soul tethered to the body and unable to act, and simultaneously the body acting on its own due to imprinted memory.
Wait, I've got it. The soul of an innocent person is tethered to the reanimated body and cannot leave, but also cannot exhibit normal control over it. (The body has a "mind of its own" so to speak, enough to allow it to shamble around and so on as Zachary Smith
describes. Not evil per se, just unintelligent and aggressive.)
It is possible for the soul to gain partial control over the body to some degree, but it can only do so by indelibly twisting itself to match the zombie's bestial nature.
Turning to the dark side, so to speak. This allows for certain zombies to speak, reason to some extent, and possess at least partial memory, but it also means those zombies become truly evil in practically every case. And those souls truly are damned.
And keeping the soul in this particular equation allows for another nasty catch-22: Either be stuck in your own body unable to affect anything forever, or regain conscious control of your body at the cost of your soul.
I'm kind of making this up as I go. Does it make sense or contradict anything?