Regarding focus, I'll admit there's a different kind of attention needed for driving vs walking amongst cars, but I don't think driving is anywhere near as intensive as you and some other posters are implying. How often do we hear about "driving home on automatic" where you get home and have no real memory of the act of doing so? Obviously there are exceptions, but often while driving, you are typically moving along at a consistent speed with other vehicles, paying them little mind unless they begin to act in an unexpected manner. Cars from the opposite direction are sometimes physically separated, but even when not they're treated the same way - an assumption of normal behavior.
Unless you're driving through a neighborhood street with cars and other visual obstacles, under the constant threat of something appearing suddenly in front of you, driving is simply not usually an attention-intensive activity. Maybe it _should_ be, but based on the number of people eating, using cell phones, etc while driving, evidence suggests most drivers don't treat it as such.
And I'll agree that if a pedestrian is well-removed from cars, such as on a sidewalk with a reasonable median between it and the road, they don't need to give cars much thought. But there are many other places where they do. The places I have the most trouble are uncontrolled intersections, which happen every block in suburbs / residential areas. None of these are metered or have marked crossings. The other are driveways into commercial areas; too many drivers pull right up onto the sidewalk, hardly slowing until their nose is in traffic, and only look left when making a right-hand turn.