I can think of some reasons
1) Foods with too many fats, sugar, salt, and high caloric foods, generally taste better than other foods like veggies or other healthy options. We naturally have a sweet tooth as a survival instinct, so we naturally crave those types of foods and satisfies our Reward System in our brains
2) Think of fast food vs. Grocery shopping. Which is cheaper? Fast food is simply cheaper, faster, and more convenient than cooking something healthy at home or eating with family. Americans are so busy and many low-income families simply cannot afford to eat healthily.
those are some reasons I hope this helps :)
