I created an account just to weight in as well. I'm pretty sure that this is simply ironic (and therefore humorous) because it's a caveman who is not living in a cave, but instead in a house. This is what the house of a caveman would look like, get it? Larson takes ideas or concepts we have some conception of (cavemen) and ascribes relatable characteristics to them which, again, makes it funny. It's not incredibly deep, just kind of subtle.