I can’t speak for everyone, but using myself as an example, I grew up in an extremely poor area and it was cheaper to go buy a box of some high fat/sugar, ultra processed food than to buy fresh food and meats. Unsurprisingly, we had a lot of fat kids in my school.This is often stated, but is objectively false.
Junk foods and pre-prepared meals are some of the most expensive items in the grocery store. Most fresh produce is dirt cheap, although there are some money traps (I'm looking at you, grapes). Frozen produce is where you start to get expensive. Walmart Supercenters sell cheap, fresh produce all over the country. I guarantee you can find items like potatoes, rice, corn, tomatoes, broccoli, green beans, apples, oranges, and bananas everywhere for extremely low cost whereas a large bag of Doritos cost about $7 (after tax) - that $7 can buy a week's supply of bananas, apples, or broccoli for a family of 4.
Not to mention the exponentially more expensive cost of getting McDonald's or KFC instead of whipping up a quick meal. I also can't prove it with a study, but I tend to notice that the vice for obese men seems to be copious amounts of alcohol with way too large portions of red meat and bacon while for women it seems to be iced coffee or frappuccinos with way too many desserts. For kids, it's because mom and dad put too much junk food, frozen dinners, and ice cream in the house... all of which are expensive, optional food and drink items. When the kids say "I'm hungry" it's okay to say "great, dinner will be in 2 hours. Have an apple."
People are electing the perception of convenience over money. Hence why I think aggressive ad campaigns would go a long way. I also think that it's more likely that people are shopping like their parents, and maybe in the 1970s-1990s it was true that instant mashed potatoes, hamburger helper, and frozen hot dogs were the cheapest way to feed a family so like father/son, like mother/daughter... but it's not true anymore.
I know anecdotal evidence doesn’t prove a theorem, I’ll just say that in some areas of the country, it is 100% true that it’s cheaper to buy the unhealthy stuff.