Nothing screws up a wasteland of shambling monsters like a group of human survivors. Every person who joins your rabble takes your exciting tale of action horror one step closer to psychological melodrama. The Walking Dead is almost entirely about human men flossing their teeth with one another's tongues. What happens is that when people form fruit loop society microcosms, they start to realize that they have to hang on to the only thing they have left: their humanity. You know why? Because when someone sits around thinking about crap like this too long, their thoughts fold in on themselves until their entire brain becomes a vagina.
Let's see if that's true by dealing with a common zombie situation. Say a female survivor is bitten. Of course, you can't shoot your friend in the head, no matter how psychopathic you think you'll be once civilization ends. So soon you're in a heated argument with your own soul and the other survivors about how to get rid of this zombie time bomb. And if I know zombie fiction, she's about to interrupt to say she's pregnant, just so you all know you're about to execute the very concept of hope itself. This kind of drama will repeat every single time someone steals a box of cookies or gets overtaken by a horde and left for dead. A few well-intentioned survivors can turn even the nicest apocalypse into a Dove Body Mist commercial.
You know who doesn't have to deal with that s**t? The feral maniac living in the sewer and becoming one with the night. Basically, when the end of the world arrives, you have two choices: Spend it as Tarzan or spend it as Meryl Streep.