I think I may have spotted a mistake in your formula.
Well, indeed. It turns out that for a good movie the audience neeRAB to be emotionally invested in the characters and what happens to them. Merely having zombies doesn't guarantee that. Even having gore isn't enough if we don't care about the people being gored.
Zombie movies tend to be made by amateurs because they look easy. Good zombie movies are rare because good people are rarely attracted to the genre. Those who are, usually have some deep artistic vision they want to express. The zombie becomes a metaphor for something else.
It's the same with the Alien Versus Predator films. The people who made them thought that all you needed was to get the two species fighting each other. They were fans, but they didn't understand why the earlier films they were fans of, worked.