Belt-conveyer runners

Along the jogging course in the park, joggers pass a training gym. When they look inside the gym they find another joggers looking outside the gym. Tens of people are running facing the window, without uttering a word, on treadmills placed in a row along the window. It is a weird sight. We are running for free in fresh air enjoying a moving view around us as we run. On the other hand, they, inside runners, are running (if you call it a running) for a charge of 500 yen in sweat-reeking air enjoying a view of outside joggers enjoying a moving view. I simply can't understand why, on such a lovely day of spring, they prefer to be indoors on a belt-conveyer to consume the same amount of energy.
When I went to San Diego, I saw a similar sight. There was a food mall with a huge parking lot. It was a warm spring night. One of the buildings which we at first thought a restaurant was actually a training gym, inside which many people, young and old, were jogging on treadmills looking outside at us. It gave me an uncanny impression but I thought "Oh, this is America. People are scared to run outside for fear of being shot." Maybe I was wrong. Many people nowadays stay inside because they like it.
Why do they like it so much? Here it is. One more thing to consider.