Prey are food for predators.
Humans have many characteristics of predators. We look like predators, we think like predators, we act like predators.
Some humans are in fact almost strict carnivores. Most of the rest of us are omnivores. Some are strict vegatarians, usually in select environments, or supplimented by modern transportation, and food processing.
Why are plants meant to be food? They are living, just as sentient beings are.