I'm surprised they don't mention the obvious practical reason. I walk faster when I'm in the city because I have farther to go. Last week, I was working in a smaller city 30 miles out. So I drove, and out in the suburbs, I can park pretty close to where I'm going. Today, I was working in the city, a mile from home. At that distance, there's a good chance I wouldn't be able to find parking any closer to where I'm goin…
"a mile from home. At that distance, there's a good chance I wouldn't be able to find parking any closer to where I'm going" Would you drive, just to travel a mile? Barely seems worth it in the best case. That's without getting into the environmental and social issues.
The bigger issue is that the "best case" is more brought by luck than anything. I've lived plenty of places where walking that mile wasn't actually feasible. No sidewalks, for example. Work that expects employees to arrive to work clean and dry, no matter the weather. Work that provides little space to store winter gear. Sometimes, the drive is seriously shorter: You might be driving a mile, but they don't allow walking on those roads, so the walk is a mile and a half.
The social issues are important things to consider depending on where you live, as they might make it nearly impossible to walk without police harassment. Especially at 2am.