Well I don’t think it’s mentioned as much as shown. They were eating dogs or something or other. They are also very slow of course so for them to capture and bring down a fast moving animal would be virtually impossible. Generally they can only capture humans by cornering them or by their sheer volume and ability to out number. So perhaps this is the reason they "can" eat live animals but then normally don’t. It would require too much effort and the movie goers want to be shocked and grossed out so seeing them devour humans is more gruesome.

You do realize that "zombies" actually exist? In countries such as Haiti, they can use kurari and various other concoctions to render a human "lifeless". It isn’t limited to Haiti either but the most research on the topic has been documented there.