One thing to consider is that Many people simply do not have the time, energy, and/or equipment to cook food from raw ingredients. These factors are common in poorer households where people are working multiple jobs, etc.
* Bananas, a fine inexpensive convenient snack.
* Buy bread and sandwich fillers, add sliced tomato, not awful for you and not time consuming.
* Tea is cheap, I like tea.
* Cereal can be good if you can find one that hits the sweet spot between a bowl of chocolate and a bowl of chaff. Some sort of granola maybe.
Most kitchen equipment except for the white goods is dirt cheap and even the really cheap stuff can last for years. Most meals take me 30-40 minutes to cook maximum.
In any-event. It is objectively more expensive to buy junk food, that includes the cheaper junk. Once I cut out the trips to the takeaway and the deli, I was surprised at how much more money I had per month.
The fact is that people tend to want to eat junk.