Because some homes are set far away from main roads(sometimes an eighth of a mile), and all houses are spaced a long ways out(usually more than an eighth of a mile where I live!), so to make it faster and more efficient for the mail men to gather and distribute mail, not only do we set mailboxes conveniently against main roads, but sometimes they're even on the same side of the street so they don't have to turn around to get the people on the other side of the street.
Also when you're driving in general you won't be getting out to push mail through a flap.