Well, one of the major advantages cited by the designers of having a city in a building was having nature close by, as well as extensive promenades. However, as you mentioned earlier, there's linear distance, and "elevator" distance.

I think part of the situation may be the emphasis on going to the gym and doing specific "sculpting" exercises, rather than braving the outdoors with its pollen, wild animals and strange people. Also, there's always the intarwebs to blame.