And there is a tonne of health benefits (and sexual perks) for those people who choose to go vegetarian or vegan. (There is also fruitarians, but those people are just plain crazy and suffer from a chemical imbalance.)
Vegans typically have less health problems, less headaches, less cancer, more energy and more sexual libido.
However for a a variety of practical reasons the vast majority of people are omnivores because...
#1. Eating omnivorously is more convenient. You don't have to be constantly checking labels to see if there is any animal biproducts in the food you are buying.
#2. Eating omnivorously can be cheaper (but isn't always). Especially compared to people who only buy "organic" fruits and vegetables which have been genetically modified. Organic is really a misnomer... what it means is its "pesticide free", but what they don't tell you is that they use other methods to kill insects... namely toxins in genetically modified plants which poison / deter the insects. However if you're not worried about buying foods that are organic (or grow your own vegetables) then going vegan is actually cheaper...
#3. Eating omnivorously is less stressful. You don't have to worry about vegan alternatives, where to eat, what to eat, who you are eating with, etc.
#4. Because their family / children / friends are omnivores and this creates problems for people socially.
#5. Because they're hopelessly addicted to bacon. (Okay, that is not a practical reason, but you get my point.)
Basically the majority of people can't deal with going Vegan because it just feels like too many rules, too stressful and not practical for their daily lives.
However there is a more practical approach to going vegan or vegetarian... I am not certain what to call this diet... its sort of a "Half Veg Diet"... or a "Veg-Curious Diet". Whatever you want to call it...
The Rules of the Half Veg / Veg Curious Diet
#1. The rules of this diet are really more like "guidelines". You can pick and choose which ones you want to follow. All the rules are OPTIONAL and can be modified as you see fit.
#2. You can make up and add your own rules. Whatever you find convenient or practical for you.
#3. Don't bother buying meat that often. ie. Only buy meat when its on sale.
#4. Avoid eating meat unless its offered to you. ie. You're at a friend's place and they offer you a burger.
#5. Avoid buying dairy or eggs (or other things you know to be non-Vegan) unless you actually need it. ie. You need milk for your cereal.
#6. Try to balance your diet towards more veggies / fruits / berries / grains and less meat/dairy products.
The end goal here is to do several things:
#1. Save money by buying meat less often and/or only eating meat when its either on sale or given to you for free.
#2. Still be an omnivore socially and still be able to enjoy the foods you love. (A bit like someone who only smokes when someone offers them a cigarette.)
#3. Not turn into one of those preachy people who look down on others for being an omnivore. (Not all Vegetarians or Vegans are like that, but there are a few preachy people out there who give the whole movement a bad rep.)
In theory the principles of this diet can also be applied to general dieting with respect to fatty / sugary foods... ie. Only buying fattening or sugary things when they're on sale and/or only eating such foods when they're offered to you and therefore are a treat.
The ultimate goal of the Half Veg / Veg Curious Diet is that you get to enjoy some of the health benefits of being a Vegan, but without going all the way. It also makes a good stepping stone for if you later decide to become a hardcore vegan.
If you do eventually decide to go the hardcore vegan route, you will likely want to learn more about veganism in general. I recommend the blog Beautiful Vegan for further reading. I also recommend the eBook Conscious Evolution available at the author's website or via Lulu.com.