You seem to need to go through a lot of gyrations and false dichotomies in order to give us the impression that having a garden is nigh-on impossible for most people. And you seem to imply that those who were not farmers a long time ago did not have gardens, though provide nothing to back such an assertion.

We're discussing a great deal of time-consuming back-breaking work, not just "a little fresh air" (hard labor to be done by people who in at least a good many cases are plopping down in front of the TV in the evening and pretending to watch through half-closed eyes because they're knackered out.)

Gardening and horticulture doesn't have to be back breaking work, it depends upon how you approach it and how much you tackle. I don't think you've done much gardening yourself, but perhaps you could inform us otherwise (and the type of garden, if you have one). Those with mulched raised beds, for example, would be able to tell you that endless heavy weeding is not a given with gardening. Nor would those who use the Fukuoka method, among others. Growing nut trees requires little in the way of heavy work, and with a smart selection of cultivars, 97% of that comes at harvest time.

Those people who spend the day behind a desk and behind a wheel would be invigorated with some physical activity, instead of just vegetating in front of the boob tube. Note that the 1918 flu pandemic attacked those between 12 and 40 the hardest, leaving a much higher percentage of the very young and the elderly. And it's not like all of the people living now are old and useless; I had an aunt who gardened well into her 80s until passing just recently.