Optimal auctions maximize revenue by assigning goods to highest bidders.
In an optimal auction, sellers can make more money by giving goods to the right people, even if it seems inefficient. When sellers can't control resale, they might not want to give goods to the highest bidders. But if resale is perfect, sellers should always give goods to the highest bidders to make the most money. This means sellers don't have a reason to give goods to the wrong people.