Because of a lack of moderation on our part. The simple fact is, humans are omnivores, not herbivores or carnivores. our bodies are designed to eat a varied diet, and, indeed need a varied diet in order to maintain health. I consider strict vegetarianism to be unbalanced, unhealthy, and extreme in much the same way as strict carnivores are. I stand by my belief that to live, we must kill, be it plant life(which has been proven to have it's own type of consciousness) or animal life, or fungoid life, or any of a number of other organisms that we unknowingly kill and/or consume every day of our lives. This is one way that we learn about death being a part of(and necessary to) life. I think anyone who adopts a diet due to an ideology is missing the point. It is putting form over substance.