1. Nails and teeth would be the first things to fall off of a decaying body, which would make a zombie pretty innocuous. Sure, it would be pretty traumatic to have a zombie gumming your arm, or pawing at your chest, but without nails to scratch, or teeth to bite, how would they pass on the virus?
2. As top predators, zombies would occupy a higher trophic level than us, and would need to eat a lot of humans to keep up their energy needs. How could they hunt down enough humans, especially when there are large zombie hoards? It just doesn’t make thermodynamic sense. The caveat to this idea though is that if they’re cold blooded and if they aren’t trying to support an energy-sucking brain, like humans do, then they may be able to survive on fewer calories per day.
3. Within a few days of death, a zombie would become a bloated, decomposing, maggot-ridden bag of gas. The human body is full of nutrients, and insects and bacteria make pretty quick work of defenseless human flesh. A zombie would be a puddle goo in no time.
4. Don’t tell PETA, but zombies would be vegan. It makes more sense for zombies to eat grass and weeds than brrrraiinnns because digesting meat requires teeth to chew the flesh and organs to make protein-dissolving enzymes. Since they don’t have a working digestive track, they would instead make use of their native bacterial populations to break down plant matter and to generate the amino acids and vitamins they would need to wander around and scare people.
5. Dogs and other predators would eat them. Sure, they’re made of rotting meat, but my well-fed Chihuahua-mix will drag month-old avocado rinds from my compost pile to nibble on, so I imagine if there were packs of abandoned pets they would start hunting down zombies for dinner. Lions, tigers, bears, ostriches and other large predators could also take a bite out of the zombie population. Literally.