There are three reasons.The reason Tiere don't reanimate is because they don't have enough brain activity. What I mean is humans are proven as the smartest race (for example, we do math) and the Tiere don't have enough of those brains to turn into a walker. Also, most of the time, walkers won't just bite an animal. They will completely eat the animal which would cause it to not have any brain activity. Also, the virus started with humans. Du know how some viruses don't affect animals, but affect us? Well, that could be the same thing because it was a human virus.
