Because they walk a few minutes per day: http://www.sciencedaily.com/releases/2009/03/090326134014.htm
According to the study, people who drove the most were the least likely to meet the recommended level of physical activity. "The idea of needing to go to the gym to get your daily dose of exercise is a misperception," says Frank, the J. Armand Bombardier Chairholder in Sustainable Transportation and a researcher at the UBC Institute for Resources, Environment and Sustainability. "These short walks throughout our day are historically how we have gotten our activity. Unfortunately, we've engineered this activity out of our daily lives."
Also, see this report by the Robert Wood Johnson Federation: http://www.rwjf.org/pr/synthesis/reports_and_briefs/issue11.html
Obesity is the second leading cause of preventable death in the U.S., contributing to more than 100,000 deaths annually and a growing burden of chronic disease. Traditionally, interventions to increase physical activity and combat obesity have targeted individual behavior change through education and promotion. There is increasing recognition by researchers and public health leaders, however, of the need to expand the focus of interventions to the environments and contexts in which poor nutrition and lack of activity occurs. This focus has spawned a relatively new body of research examining the role of the community “built environment” in promoting or discouraging physical activity.