From my understanding, you don't "own" a game you bought on Steam, you just own the license to play it. The game file without modifications is protected by DRM, and only works when it's launched from Steam with a valid license. Notice when using the same account on two different PCs, Steam would force quit the game when you try to launch the same game from the other PC.

In a closed system like Steam, sure, it would be relatively easy to regulate the buying and selling of game licenses since you're doing it all under Steam's system. When Steam detects a license transfer or however they want to implement it, they can easily disable access for the seller and enable it for the buyer.

But if the game file is DRM free, then it's the same as downloading pirated movies, there would be no guarantee that the seller has no access to the game after selling it. No way to regulate it either. Hence, endless copies.