There are so much reasons behind that calculation.
Main thing was that junk food was too costly for most of the people that time. If we take the pre-1900s, most of the people were not rich enough to get protein and ate food that would not been considered healthy and nutritious.
The exception was the farmer’s family because they worked hard and were skinny and trimmed in shape.
Junk food has recently become available and cheap enough to eat routinely.
In the past, there was not much money and people were not ready to spend money for frivolous things.
The other important thing is the daily routine. If we look at it, firstly people tended not to sit as often like we do now. It was rare for a family to spend an entire evening watching TV.
They sometimes played games or listened to the radio, but during reasonable months, people tended to be outside more. Also more people walked to work or took public transportation and then walked, usually at both ends.
In the 50s and early 60s, although junk food was available, it was comparatively expensive than this era in some sense. Junk food price dropped after corn oil and corn syrup became available as a very cheap fast food ingredient. Packaging was also a lot more expensive. As junk foods became more profitable, they were heavily advertised. Consumption increased because most people found them convenient and delicious.
In recent decades, the preparation of junk food changed. Saturated fats were largely replaced by partially hydrogenated vegetable oils. Such oils are rich in trans fats, which are now known to stoke inflammation and lead to weight gain.
So now the story continuing…