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.
In Day Of The Dead
, one of the scientists experimented on a Zombie-- "Bubba" or something, his name was-- and found evidence of consciousness. I'm not sure if this was ever followed up. I must catch up on the Romero movies.