Convenient junk food is quite expensive compared to “real food” like meat, uncooked pasta, fruits, vegetable, etc which are healthy but require some preparation.
The argument then goes: “well, ok, but poor people don’t have time to cook”. Which again rings false. The poor people I’ve known have an abundance of time and are not working 3-4 jobs to make ends meet.
And prep is easy, I guess my tastes are simple - beef, eggs, salt maybe a litle hot sauce. I can cook 3 pounds of ground beef and add about 10-12 eggs in a 12" fry pan and have 3-6 meals in containers in about 45 minutes.
I have some food allergies and cook everything from scratch at home so I know it can be cheap to cook yourself but there are some processed foods that are just cheap for the calories especially things on sale.
Other factors like limited transportation and space in a backpack or bag to carry home can make bulk savings harder.
Yeah you can eat rice and beans for cheap, but that’s not really a completely nutritious diet. I’ve eaten it a lot!