Zombies are considered to be part of the undead (not alive), often will-less and speechless bodies that have been reanimated by spirits.
Being a Christian isn't to be compared to being a zombie, for the Holy Spirit resides in us and Jesus has imparted his righteousness to the believer/Christian. We still have our speech and will, we either choose to submit to Him(Jesus) or we do what pleases our "flesh."
The reason there are no zombies roaming the earth, because there is a God, heaven and a hell. Those who rejected Christ are in hell, those who sought Christ for salvation go to "heaven."
Those poor people who are in darkness and have experimented with various "potions"
that can make one appear like a "zombie" (such as those cases reported in Haiti)
have damaged not only their minds but their bodies also. I know some people
have been made into a "zombie" by a revengeful person/rival and in some cases the person is performing a voodoo ritual or are trying to defraud and have caused damage to themselves by ingesting a potion that has too much..... in it(sorry I won't reveal the substance.)