They're Kinetic Catastrophes
We're all mechanical animals of sorts, our locomotion made possible by the links between muscles, tendons, skeletal elements and much more. When part of that system goes awry, we don't move much, if at all. That makes it all the more puzzling that so many modern-day zombies are perfectly capable of movement even when their flesh and bones are dangling and flopping.
And yet, here are all of these zombies staggering around (sometimes with frightening speed), seemingly oblivious to the impossible physics driving forward their rotting, shredded muscles and broken bones. That's even before you consider their lack of brains.
The human central nervous system controls all of our muscular activity by firing electrical signals from the brain to muscle cells, which twitch in response to the gray matter's commands. Many zombies appear to suffer from massive head wounds that would render any brain completely non-functional, making the idea of forward motion all the more implausible.