For one, treadmills are expensive. Each one costs thousands of dollars, requires expensive upkeep, and drain electricity. Mother nature frowns every time a human buys a treadmill.
Secondly, treadmills do a terrible job of replicating what it’s like to run outside, for which humans are best-suited.
Thirdly, treadmills are an icon of modern convenience. Don’t want to be too cold or too hot? We’ll control the temperature for you. Don’t want to feel unsafe? We’ll give you a secluded space. Treadmills help you seek comfort, and hence, decrease your tolerance for discomfort.
Lastly, most people don’t even like using treadmills. It’s not surprising that treadmills originated in prisons, and were used to punish unruly inmates.