What's your best guess tRg?
I say:
(a) Sheer laziness. And why are we lazy? Good reason: a zillion years ago, even several hundred years ago, life was a lot harder and food much scarcer. You had to work hard enough hunting and/or gathering food just to survive, you were pushing the limits of things like available calories and your ability to recover and do it again the next day.
So we developed ("we" including our earlier animal ancestors, before God made Darwin) the instinct to park our asses and conserve energy outside of the necessary work. Once upon a time "outside of the necessary work" may have meant a few hours a day but of course we have a lot more ass-parking leisure now.
(b) Herd instinct. Of those who manage to overcome the ass-parking instinct, most of 'em don't run... or if they do, they just "do cardio" for 20 minutes on the treadmill. Because most folks don't do anything without a group, and the answer to "where do people exercise" is apparently "the gym, silly."
(c) It really *is* hard at first, even jogging a couple miles, at least for an adult trying to start from scratch. It's harder physically than say riding a bike, and also harder psychologically - with the bike, not only can you coast, not only are you seated, but there's also this comforting notion that you've got a machine helping you. In a sense you just gotta push the pedals and it does much of the work. You're not all alone, exposed, dependent on only yourself.