Originally Posted by BlankCanvasDJ
This. At least, historically, that's been the case in zombie fiction. We don't do things at full strength because consciously and unconsciously, we are worried about things like tearing muscles and breaking bones. Zombies don't have that reflex. If they want to pull on something, they will do it until their arms come off or that something gives. Since it takes less force to break skin than it does to rip an arm out of a socket, they can and will tear into your flesh.
Plus, since it's relatively thin and zombies are always grasping and clutching at things, I would imagine the skin on their fingers would be some of the first to go, leave nice, hard, potentially sharp bone.
While I can buy this explanation, it also leads to questions like - why, during the fifty million times that somebody's leg gets grabbed by a walker during the game, aren't their legs just ripped clean off in one go? If they have no internal strength inhibiting desires, it seems like they could tear your legs or arms off like pulling a turkey's leg off (happy almost Thanksgiving, everyone!).
Just a thought. Like somebody else said, you can't bank on any serious level of realism in this kind of environment - it would just make everything boring.