Because we are omnivores. If an organism cannot survive without meat (a pure carnivore) then there simply is no option. For example you can raise a dog purely vegetarian. A cat dies (you can make it eat vegetarian food, but it still won't survive).
Very few animals do. Also note that canibalism is dangerous for any organism.
PS: You can annoy most vegetarians far easier... Ask them why they wear leather... (shoes, belts,...)
-- Sure, you can "raise" a dog that way, but in its natural state, it WILL eat meat, if any is available. They are instinctively hunters, as are their cousins, the wolves. Indeed, IIRC that we are the only omnivores who can/do "choose" to eat meat or not. But thanx, tho', for pointing out a way that we can help rid the world of cats
-- Very few animals "do" what
?... I was classing humans here as a higher lifeform than the rest of the animals (not that the PETA types would agree, but so what...). Thus, my point was that the lower animals eat other animals; if we were to do likewise it would be somewhat
analogous to cannibalism.
-- Good point; wish I'd thought of it originally... Although I think your point would be equally valid w/ PETA folx, too (not all of whom are necessarily vegetarians).
-- Also, we could challenge 'em about eating eggs, consuming dairy products, hanging bug strips on their patios ("hey, insects are animals too!"), etc. etc. etc....
...btw, what's the diff between a vegetarian & a vegan? IS there any difference?...