So what if the world has ended. Big deal. Life goes on.

Now that humans aren't the most sentient race, they were overthrown by... blood. The remains of the civilization is nothing but a bunch of random obstacles for them, as they try to grow their own mass by consuming their own kind. You can't blame them really. It's just the bigger you are, the easier it is to deal with humans.

Oh yeah... humans. Uh, so you see, while not sentient anymore, they are still apex predators.