I think there are LOTS of reasons.
The NYT article blames the farm subsidy program for encouraging the farmers to produce as much as possible and then sell it cheaply.
Then restaurants and prepared food manufacturers find their raw materials so cheap that adding portion size adds a negligeable amount to the cost.
Then voters are so niggardly about funding schools that school boards have to scramble for money so they put vending machines all over selling sodas and chips and whatnot.
When I was growing up in the 40s, there were relatively few cars in my lower middle class neighborhood. How few? Well, my family didn’t get a car till my senior year in HS, that was 1953.
For more evidence of that, we kids played on the street. During the summer, nearly all day and the rest of the day from the end of school till dark. Either touch football or a baseball family game called boxball. During the daytime, there were no cars in our way and a car came down the street forcing us to move maybe every 10-15 minutes. This was in 1945 and by 1950 there were too many parked cars.
There were no TVs, and we played out all the time. What I see with my grandchildren is that their lives are full of scheduled activities. This means getting into a car and being driven some place to get instruction and play and then getting back into the car and driven home. They are driven to school because walking is perceived as dangerous (and maybe with all the cars fighting to get into and out of the school yard, it is). They get maybe a half hour of real activity in an hour and a half and they get this once or twice a week. Compare with when I was growing up.
Even after we got a car, we used public transportation for many things. This involved walking several blocks to the trolley and bus. When I first moved to Montreal, it had pretty good public transport, but while the Metro is still good most busses run only every 20 to 30 minutes and often omit a scheduled bus when a driver doesn’t show up, so they are no longer reliable either.
Many suburbs don’t even have sidewalks nor any place to walk if they did. And where there are public busses, they often run once and hour if that often.
People used to go out at nights. Now they stay home and watch TV. The effect of TV (and, admit it, computers) on kids is even more profound. My grandchildren do not go out to play. I am not sure they would be permitted to. And they live in a cul de sac where it would be very safe.
Then there are all the TV dinners, prepared foods and the like. I suspect they are higher in calories than scratch foods, because fat, sugar (and salt, BTW) sell.
I think it is all that, and more. I haven’t even mentioned labor-saving devices (till now).