Others here have already brought up the wear and tear running can have on the human body, but there's a psychological factor as well. For many, walking is more

Others here have already brought up the wear and tear running can have on the human body, but there's a psychological factor as well. For many, walking is more enjoyable than most other exercise. Some people can do strenuous, repetitive exercise and not get bored or feel miserable, which is why a certain number of people can manage to go to the gym regularly and stare at a wall for an hour as they run on a treadmill and lift dumbbells. Others give up more easily in reaction to stimuli that isn't highly dopamine driven for them. For those people, if they can instead keep up walking on a regular basis, it makes more sense they stick with that rather than effectively punish themselves by running.