Some comment here on HN a month or so ago clarified this for me. It's more efficient to have a delivery truck go around to ten different houses than it is to have each of ten different vehicles drive from their houses to a central location.
Now if only there were some way to do that with people, not just with stuff (cough mumble public transit cough).
Now if only there were some way to do that with people, not just with stuff (cough mumble public transit cough).