Have you ever thought about this question? Did you ever wonder why a creature that is dead with a digestive system that no longer works would need to eat? Or did you just take it on faith that that was how the creatures acted?
Cannibalism is incredibly taboo within our society. We view it as the most abhorrent act one human can do to another. It is primal and barbaric. Only in extreme cases of survival is it viewed as tolerable. By making zombies cannibals, it shows us how monstrous they have really become.
Zombies represent the worst-case scenario of what humans can devolve into. They are driven by pure instinct and the most base of human needs. By making zombies cannibals, it makes them the most reprehensible and vile creatures to ever exist.
But there is an incredibly thin line between zombies and humans. After all, they used to be us. Perhaps another reason to make zombies cannibals is to make us look at ourselves as a society and question all that we consume.