Lack of overall health care insurance is one reason. In the US, you better have a good health care package at work, or be poor enough to qualify for Medicaid, or old enough for Medicare in one of the wealthiest countries on planet earth.

If you fall between the cracks, then some say it's cause you are too dumb or lazy to work hard enough to be able to pay outrageous private care insurance premiums.

Can't discount the American diet, too. Tasty foods high in fats and salt readily available at fast food venues. Removing PE and recess for energic play in our schools is not helping either.