Economies of scale seem like a large win, here. If you own a thousand trucks, your per-truck maintenance costs are less than if you own one truck. Companies t

Economies of scale seem like a large win, here. If you own a thousand trucks, your per-truck maintenance costs are less than if you own one truck. Companies that don't want to own and maintain a fleet of trucks will outsource to those who do, but it's simpler to deal with one external entity that can supply trucking than ten or a hundred such entities.