Go down the frozen meal aisle where I'm assuming most unhealthy people shop and it's far more expensive than buying the raw ingredients. Sure it requires more effort to cook from scratch, but you can't throw money at people not willing to put in the work and expect better results.
- Extra time required to cook
- A home with a functional refrigerator, oven, stove, etc.
- Time and transportation costs to and from a grocery store. Remember that poor people tend to live much further from a grocery store than you're used to.
- More money required up front. If you have literally $20 in your bank account, are you gonna spend some of it on bus fare to get to the grocery store an hour away, buy $10 of food, and come home, or are you gonna get a $1 hot dog from the 7/11 down the street?
[1]: https://patch.com/new-york/new-york-city/nyc-groceries-are-m...
Something that I think gets missed in this is time value. If you're poorer, you likely have less free time, and that cuts into things like shopping for groceries (doubly if you're one of the 23.5 million that live in a "food desert" [1]) and meal prep.