Cities are populated by people, not cars. As point A and point B become closer, people are less likely to drive.

If your mailbox is attached to your house, you can lean out your front door to get your mail. If your mailbox is at the end of your 20 foot driveway, you take a few steps to get your mail. If you live on a farm, and your mailbox is down a several hundred yard driveway, you might hop in your side-by-side UTV. If your mail goes to a Post Office Box in town, you might hop in your truck and pick it up while running errands.