Because they are delicious. Life is too short, not to enjoy one of life's simple pleasures. Obviously moderation is key, but it's silly not to take advantage of being human.
It's like telling an alcoholic to just have one glass of wine a day-- yeah that would be great if it works, but it's not really helpful advice for some people.
You could get most of your calories from table sugar and not gain weight if you do the math correctly. But this diet will eventually severely damage the hormonal regulation in your body.
It's not that much of a stretch to see it that way either: alcohol is nutritionally barren. And instead of raising blood sugar, it lowers it, thus rendering one prone to cravings for poor carbs.
The problem is that most diets (in America at least) are made up almost entirely out of carb heavy foods with little macro/micro nutrients.
Personally I shoot for a 45/30/25 (protein/fat/carb) spread which works best for me.
If I had a final meal wish, it would be some crusty bread, and butter. I'm NOT giving up carbs. My coworker does this, and only eats a plate of meat, and fats everyday. It sounds absolutely miserable.
But as someone currently watching his parents and grandparents suffer from the long-term effects of a lifetime of obesity and poor diet, these things can have long term consequences that don't catch up with you until it's too late.