I always looked at my feet while walking as a kid. People sometimes thought I was depressed, but I was just always thinking, and reducing visual noise in service of that.
When I started walking for exercise I pulled in a lot of the scenery. As I started trying to walk faster, and over the same old ground, my vision pulled in as I was concentrating on proprioception. On a leisurely jaunt you don't really have to think about what your legs are doing. When you're hauling ass it begins to matter if your left foot is landing a little different from your right, and how big that heave in the sidewalk is ten feet in front of you.
I wonder if it's the same as looking up and to the left when thinking?