But if you put in the time to cook for yourself, it isn’t

I already addressed that, and you have conveniently ignored it. Cooking for yourself takes time. Time is a commodity that poor people often don't have nearly enough of. If you're poor, you're going to tend to have a longer commute to get to work, and you're more likely to have more than one job that you have to juggle a schedule around. You're more likely to live in a neighborhood where you don't have ready access to grocery stores at all.

When I lived in Chicago, the last neighborhood I lived in was poor/working class. The closest real grocery store---not a corner store that had a couple of bananas and some slightly soft apples--was about two miles away. If I didn't have a car, that would have been a pretty long walk, or a 30 minute bus ride with one transfer. Public transit from where I lived to where i worked? About an hour and a half one way, by bus, train, and then a 2nd bus. With an 8.5 hour day, that means that I'm away from home a minimum of 11.5 hours. If I can get up, grab coffee, get a shower, and be out the door in one hour, that's 12.5 hours for my day so far. When I get home, I still have daily cleaning, laundry, etc. Best case scenario, if I don't want to get anything else done in a day, that's 3.5 hours at the end of the day before I have to be asleep. If I'd had a second job instead of coming straight home, well, there goes sleep and any time to do general daily housework. I'm certainly not going to have time to go to the gym, or take an hour run in the morning.

I made gyudon for myself tonight; it took about an hour and a half between prep time, cooking, and clean up, give or take. I used top round (it was cheap at Costco, and is very lean). Between all the ingredients I used--the top round roast, onions, rice, sake, soy sauce, hondashi, togarashi, ginger, and eggs--I probably spent about as much as a super-sized meal at McDonals, but it took me 85 minutes more time. And that's a pretty simple meal.