More relevant questions: - Why don't we stop subsidizing corn so much? - Why don't we teach effective nutrition and cooking in school? - Why don't we teach effective exercise, sports, and fitness in schools? - If we can ban alcohol and tobacco sales to children, why not things like added sugar, hydrogenated oils, corn syrup, etc? - If we can ban alcohol and tobacco advertising to children, why not products with added…
"Why don't we teach effective nutrition and cooking in school?" We do. It's called home economics.
"Why don't we teach effective exercise, sports, and fitness in schools?" We do. It's called gym.
"If we can ban alcohol and tobacco sales to children, why not things like added sugar, hydrogenated oils, corn syrup, etc?" Go tell a 6 year old they can't have cake on their birthday.
"If we can ban alcohol and tobacco advertising to children, why not products with added sugar, hydrogenated oils, corn syrup, etc?" Lobbyists
"Why don't we limit advertising on fast and junk "food"?" Lobbyists
"Why don't we provide healthy lunches to school children instead of using them to subsidize agricultural conglomerates for low quality food?" We kinda do: http://www.fns.usda.gov/nslp/national-school-lunch-program
"Why don't we stop subsidizing fossil fuels so much so people would walk and ride bikes more?" How many times do I have to say this: LOBBYISTS.
"Why don't we align our medical system with prevention instead of expensive and risky cures?" That's a good question, actually.
EDIT: I like falcolas' answer: Money. Long term care is more profitable than prevention.