Unfortunately, diet is often not a simple choice. The price of calories has gone down significantly for mass-produced, less-healthy food, whereas healthy food (like fresh fruit and vegetables) are nowhere near as cheap.

Look at fast food restaurant menus and the processed/frozen food section of your supermarket and it is 50% cheaper to feed a family of four that way (and perceived to be faster) than it is to use fresh, healthy ingredients.

Then factor in less-developed countries, where access to food is a major issue (today, it was announced that ~85,000 have died in Yemen due to starvation) and it becomes clear - for a very large portion of the world, diet is not a simple choice.