I'm basing my logic off biology. First, Zombies don't drink. Just standing outside, doing nothing at all, you lose water. Your basic motor skills require hydrated muscles to move. Zombies exposed would shrivel up and become immobile in very short order. Second is parasites. I assume that Zombies have no immune system since they are literally rotting as they go. Microbes on your body right now, and in the air, would begin to break down the zombies bodies. The connective tissues would weaken, and with no healing mechanism, the zombies moving would exacerbate the wear and tear.
Let's assume they can eat and somehow it heals them to some degree. What happens while they are standing around once all the people are dead or zombies? They don't eat each other. Being without a food source they would die, eventually.
When you consider things like they have no warming or cooling method, extreme heat and cold would speed up decomposition or freeze them solid respectively. Both would break down cell walls and further damage tissue. The tissue is unusable in a matter of weeks, and that's to say nothing of insects and the damage they do on rotting flesh.