Evolution

It's certainly a great example of evolution driving diversity to fill environmental niches, and there's no denying that it's likely to be the only reason that we few humans still exist, but I still find it creepy and weird to watch the ghouls hunting down and eating the zombies. Ghouls were just a myth and a part of popular roleplaying games as far as I knew before, but then so were zombies. Now it turns out we've got a thriving ecology pretty much world wide based on humans becoming zombies and ghouls eating zombies.

It's true ghouls will generally only eat zombies that are less than a week old or so, although they become considerably less picky when they get hungry, but since ghouls move almost as fast as humans and even fresh zombies can't out run healthy humans that means all we've got to do is cull the old slow zombies. Easy enough as long as you don't get cornered by a horde of them.

So now there are a few pockets of us humans. We encourage the ghouls to cull the zombies, and we apply a very strong evolutionary pressure to any ghouls that try to eat humans. There have even been some rudimentary attempts at domesticating ghouls but most people don't find that very appealing for long. If nothing else, the smell is...discouraging.