Most animals, including carnivores (like lions) live twice as long when kept in captivity and fed a regular diet free of parasites and diseases.

That is, most animals eat what they do because that is all that is available to them. But make no mistake: they’re often stick, and wind up suffering and are in poor shape before they are killed either by the diseases they carry, or by another predator.

We have deer in our back yard where I live—and the older ones I see are routinely covered in sores and often don’t look like they’re in the best of shape. In the wild White-tailed deer live 2 to 4 years on average, lasting up to 8 to 10 years if they can avoid predators, disease and hunters.

In captivity they live up to 20 years.

So many of these "why is X okay for cavemen/animals/whatever but not for us" can be answered with: it's not okay for them either, you just don't realize how privileged you are with the life expectancy, child mortality rate and general well-being that you enjoy.