No one said every animal, just humans

Yes, I'm pretty sure a lion would die if he tried eating only plants, while I'd imagine a cow would die on a meat diet. I'm less well versed on non-human animal diets though, as I've never needed to give it a lot of thought.

I do know that humans can be very strong, fit and healthy on a plant-based diet, as confirmed by the world's medical institutions, and demonstrated by vegan athletes of all types.