As we all know the main weapon of a zombie is its mostly flat human-looking teeth which are in fact far denser than human teeth. The teeth are supplemented by a supernaturally powered mandible and the four major muscles of mastication that produce an iron grip. As a zombie rots slowly it seems that the teeth, gums, and the various muscles of the jaw region rot the least. Second only to the eyes, aroma chemoreceptors, various nerves, and the brain itself. Only destruction of the brain ends a zombie’s living dead existence.
Human flesh, and blood fuels the supernatural zombie virus. In turn, the virus slows down necrosis in the bodies of the walking dead. Much of the human fuel goes to the areas mentioned above so these mindless killing machines can keep on with their slow death march. In fact if a zombies teeth are somehow destroyed they are the only body part that will grow back. When the dead are raised with their rotting teeth, or no teeth they will grow a new set of chompers.
If the teeth rotted the same speed as the rest of the body then they’d fall out quickly, and the zombie would be a minimal threat. Losing it’s eyesight, ability to smell live humans, and the instinctual human hunting abilities in the brain would render a zombie useless as an extermination method by the forces of evil on this Earth. Raising the dead as a weapon against the living via magic, or converting the living into living dead drones via the paranormal zombification virus has always been a special favorite of the dark denizens of destruction.