As far as I know, they could generally all afford as much food as they wanted.
Unless your great grandparents were rich, this seems unlikely. People massively underestimate how much worse and more expensive food was in the recent past. In the 1950's, people spent a lot of their time thinking about how to get cheaper food, and eating food we would consider to be not particularly good (pretty much everything canned).
Similarly, the best food you mom knows how to make is probably not representative of what your family was eating regularly in your grandparents' generation.
Going to the gym for 20 minutes every other weekend burns more calories than a 9 hour shift cooking food every day.
I think this is incorrect by a huge margin. Working out actually doesn't burn very many additional calories at all.
Hmm maybe you are right about my great-grandparents food availability, I should look into it more.
The second quote was meant to be a joke.
One thing to consider is food expenditure over time:
People spent twice as much on food in 1950, despite eating out half as much and mostly cooking foods we'd consider extremely cheap today.
Ultraprocessed food is a product of an optimizer that cares about a proper subset of human values, and Goodharts-away the others.
When you're cooking food for your family from scratch, you get to decide what to optimize for. You can consider variables like satiety, nutrition, taste, and cost; and decide for yourself what function of these you want to maximize.
It would be a surprising coincidence if the function that home cooks maximize, turned out to be identical to the function that the processed food industry maximizes. The processed food industry doesn't have the same incentive structure as home cooks. It doesn't live in your house and take care of your kids; it lives in a competitive free market. What it can optimize for is constrained by market economics: it is capable of caring about human values like satiety, nutrition, and taste only insofar as these matter to selling more food.
For instance, home cooks hardly ever try to reduce satiety; but industry often does: "once you pop, you can't stop"; "less filling, tastes great" are ad slogans for snack and beverage products that represent something that the market values: you will eat and drink (and buy) more of this, specifically because it is less satiating.
But the correct satiety setting for maximizing sales is not the correct satiety setting for maximizing health.
No new evidence is presented.
What is it about processed food that's so bad for you? The slicing? The smashing? The pressing? The freezing? The drying? The boiling? The canning? The conveyor belts? Is it simply filled with malice by the people who create it?
How come none of my 8 great grandparents were fat but 3 of my 4 grandparents were? They all sat at desks or worked in factories or took care of the home. They didn't have terribly different lifestyles. As far as I know, they could generally all afford as much food as they wanted. (By the way, my 4th grandparent has lived her whole life in relative wealth on unincorporated islands.)
I would choose my mom's soup over chikfila soup every time; I would choose my mom's burgers over McDonald's every time. I wouldn't say that fast food or frozen food or canned food or prepared meals are better or more palatable than homemade food at all. And my grandma was a better chef than my mom and my great grandma was a better chef than my grandma. Food is getting less palatable, not more.
What good explanations are left?
We all know that higher "socioeconomic status" magically makes you healthier and skinnier. Going to the gym for 20 minutes every other weekend burns more calories than a 9 hour shift cooking food every day. But perhaps there is a second factor. If your spouse doesn't work a job than they'll have more time to cook. I posit some kind of magic enhancement mechanism is going on here, where if your parents or spouse cook breakfast and dinner then you get a boost to the inexplicable wealth-health magic.