Want to know why those steam deals happen in the first place? Because people can't resell and trade games to people. If people want a game on steam, they have to pay whatever the developer or publisher is charging for the game, meaning money is always going directly to the developer and publisher.
With such extra security and guarantees on their payments, it's no wonder that they will often get aggressive in pricing on some of their older games by selling them for $15, $10, or even less, because they can make a few extra, guaranteed bucks on the side, because there is no used games market to exclude them from reaping those benefits on Steam. The fact that nobody or so few on GAF, a site where we are, I guess, suppose to know more than the average mainstream gamer, gets this is simply mind boggling.
You get those deals on steam because there are no used games or game trading, period.