Because why should I forcefully change my own species evolutionary path from the past 100,000 years and stop eating meat altogether? Animals are there to eat one another. My species happens to be quite superior thus enabling it to sustain itself through the killing of other animals. I do not condone the senseless killing of animals or the waste of food, but I was designed over the centuries to eat plants AND animals. I'm not about to deny my ability to do so and my enjoyment of eating said animals.

EDIT: That didn't answer your question. Evolution isn't something that's "civilized". It's just the way we are.