Because we eat crap food, we get no vitamins, our gut bacteria are totally out of whack, are neurotics stressing out over everything or are complete couch potatoes, and rarely get enough physical activity. Mainly it's because we have become a nation of addicts. Not just opiates or meth or weed either, but from everything from the internet/gaming/social media slackers to the always-have-to-be-busy type.
I see it from the 22 y/o pudgy blob of lard living at home who plays video games all day, to the 50 something hovercopter mom/grandma who runs herself to death trying to meet every last whim of their grown children and grandchildren. We are downright belligerent, stubborn, and willful in defying basic balance and common sense in taking care of our health. We don't do is what we're supposed to do when we should do it, and tell ourselves it's ok because we had yogurt, a banana, and a granola bar for breakfast, and therefore are eating healthy.
Like economics, human nature, and many other things in this world, people want to cheat reality and get away with it. As Johnny Cash sang it - you can run on for a long time, but one of these days God is going to cut you down. Many people feel sick because they make no real effort to get better and think a pill will solve it.