Zombies don't make sense. They don't have working digestive systems, they keep going without food, water, blood pressure, oxygen. They are rotting away but can bite through denim. They have no reason to be stealthy but sometimes are. They can smell the living...over their own stench? They can and cannot see well.
Here's a great article by Cracked on why zombies would be awful:
I think you just gotta embrace that they make no sense. They exist to tell a story. They act however they're supposed to act to scare you. There are a few rough guidelines, but outside that, the writers have a lot of freedom.