I'll repeat myself: It's not like I sell my game to Gamestop and suddenly 5 used copies appear out of thin air. Like Hodgman said, it's one copy, one user. Only one person can use that disc and play that game at a time.
In fairness, you're right there is a difference. Used games are actually worse than piracy if we use money here.
New game sale: $50
Used game sales total: $175
Total sales: $225
Publisher profit: $25
Gamestop profit: $125
So Gamestop makes 5x as much as the publisher. Now let's imagine that instead of buying used games, the people pirate %50 of their games and buy 1/2 new.
New game sale: $150
Used game sales total: $0
Total sales: $150
Publisher profit: $75
Gamestop profit: $30
Well, I see how that sucks for Gamestop, but even people pirating %50 of their content is better than having a used market. The publisher makes 3x more if people pirate half of their games than if they get them used. Bear in mind, the average used game passes through six owners over the course of its life.
It's pretty darn difficult to defend used games.