Because they know that not all of your food requirements can be acquired through vegetarianism. You can live and live healthy through a vegetarian diet... but you can't get all of your nutrients from vegetables.
Veggies give ya gas! And besides, give up MEAT? Are you kidding??? If God didn't want me to eat meat He wouldn't have blessed me with incisors and canine teeth. If He wanted you to be a vegetarian He would've given you a mouth full of molars, and four stomachs like a cow.
Humans are carnivores. Many things separate carnivores and vegetarian animals. A few are: Canine teeth, our digestive system, dietary needs and one very interesting fact. Carnivores have eyes facing forward (for stalking and hunting prey). Vegetarians have eyes on the side of their head (to watch for carnivores) and because their diet doesnt require hunting. Supplements can be used by vegetarians but it is never the same as the "real thing". Most of our meat is not as healthy as it used to be but good meat can still be hunted or purchased.
Because humans are omnivores! Biologically speaking, we're meant to eat both. But with today's knowledge, I think you can be perfectly healthy by being a vegetarian. It's just more difficult since you have to be picky about what you eat and choose the right things that will make up for what you lose by not eating any meat. So for one, it's just easier being an omnivore. Second, meat tastes good. And certain kinds of meat can be healthy. Fish is GREAT for you, chicken is alright, but everyone should probably stay away from pork or red meat and just have a turkey burger instead.. So if you don't overindulge in meat, and you eat the right kind of meat, there's no reason to be a vegetarian except for the fact that you're eating living creatures. But even then, some might argue that that's just how nature works. Those at the bottom of the food chain get eaten by those at the top.
Because there are essential vitamin in ALL meats including vitamins A, B, C, D, K and B12. But the most important are zinc and proteins. This is why most vegetarians get very sick because they lack most of it. Unless you take supplements to substitute the loss of meats, you can get very very sick. Believe me, I have spent ENOUGH time in the hospital for various problems and I am only 13. (I am no longer a vegan)
Maybe a lot of people are in strength sports or have occupations that require muscular strength. I mean seriously, after 8+ hours of doing hard construction, you're not going to rebuild those worn-out muscles with tofu and lettuce.