Some "superfoods," exotic fruits, nice cuts of meat and fish, and certain vegetables are expensive, and that's a problem, but its a problem at the margins. The basic building blocks of a healthy diet are all widely available and affordable.
Some quick googling says the avg price for a dozen eggs in the US is $1.48. A can of tuna is $0.77. A pound of bananas is $0.57. A pound of potatoes $0.75. Dried beans/lentils/rice are even cheaper.
I understand there are complicating factors. You need a place to cook, and a big mac probably generally looks more enticing than canned fish, especially if you've spent all day on your feet at a job you hate. But these are separate problems, unrelated to the food supply chain.
Related:
"How to escape The Pleasure Trap!" By Douglas Lisle and Alan Goldhamer https://web.archive.org/web/20150430050047/http://drfuhrman....
"Supernormal Stimuli: How Primal Urges Overran Their Evolutionary Purpose" by Deirdre Barrett https://en.wikipedia.org/wiki/Supernormal_Stimuli
"The Acceleration of Addictiveness" by Paul Graham http://www.paulgraham.com/addiction.html
"The Lifestyle Cure" https://tlc.ku.edu/
From the last: "We were never designed for the sedentary, indoor, sleep-deprived, socially-isolated, fast-food-laden, frenetic pace of modern life. (Stephen Ilardi, PhD)"
When I was younger even though I lived on a major street downtown in a city, there were no grocery stores around. Plenty of fast food restaurants with $1 menus though. Cheap, quick, and easy, but not all that healthy. Actual grocery shopping was a once or twice a month thing - bus ride out to the store in the strip mall, cab ride back, 2+ hours total and a lot of money.
One of the things I love about my house now is that there's a full grocery store right at the end of the street (along with other stores and a restaurant a block over). But most people don't have that.