As someone who is in the process of writing her own zombie novel, this is a bit discouraging. Will people learn that I'm writing about zombies and automatically brush it off as fluff or a plotless gorefest?
The horror genre has gotten a really bad reputation over the past few decades. It's a great genre for aspiring filmmakers because you can make a great horror movie without spending a lot of money (The Blair Witch Project, Paranormal Activity, etc.) There are plenty of knock-offs of great movies that fail to capture the fear, characterization, and atmosphere that made their predecessors a success. But all of the horror movies that are even moderately successful have something in common: They all make us care about someone. Even if they're just lost college students (The Blair Witch Project) or the innocent babysitter (Halloween), horror movies (and novels) are just like any other story: They're about the characters.
Even these zombies have a story. (source)
In the book Everything You Ever Wanted to Know About Zombies, author Matt Mogk frequently mentions (and praises) director George A. Romero. And why wouldn't he? Romero is held responsible for singlehandedly defining the modern zombie movie. Nearly every zombie story (book or movie) told in the past three decades has been at least partially influenced by Night of the Living Dead or one of its sequels, subconsciously or otherwise. There is a reason why Romero's movies are so popular. As Mogk says in the book:
"Romero is clear in all of his films that the zombies aren't the real threat and never really were. To him, the root cause of mankind's demise at the hands of the undead is our own selfish agendas and unchecked ego. Ultimately, we are the tools of our destruction, because we're not able to work together to eradicate the lesser evil: the zombies." (page 141)
Every horror novel, every horror movie, every book or movie of any other genre, is not about the external factors. Zombies, evil warlords, sparkly vampires...none of that matters. If you don't have someone or something that an audience can identify with, your story is going nowhere.
So the next time you pick up a popular horror novel (or even a romance novel or science fiction novel), don't dismiss it as fluff, full of gratuitous sex or violence to keep an audience interested. If the audience wasn't interested in the characters, the story never would have gotten noticed in the first place.