Fun trivia knowledge for ya: Gamestop makes a 50% gross profit from used game sales compared to the 21% they received from new games. At last count, over 53% of all games Gamestop sold were used, and it accounts for around half of Gamestops overall profits, and that's counting hardware and other such things.
I use Gamestop as my example because I was able to find some sales data for them, but every game retailer is telling the same story.
Now consider you can't resell most modern PC games, once they're sold they're sold, that's it! And what few games you could easily resell are difficult to tell apart from those you can't, how would a bored gamestop employee be able to tell the difference? They can't, and so the vast majority of the time there are no used PC games.
Then when you consider that PC gaming is only around 20%ish of the new games market, a market which is already smaller than the used games sales, and suddenly you begin to understand. Now new console games when sold have a chance of being traded back so that Gamestop could resell them at a higher profit. It's an investment to stock them. PC games nosomuch. Taking all that into account you begin to understand why these retailers choose to stop stocking PC games; it's a simple effort vs. profit.
Meanwhile go to any store that sells games, but not used games, and you'll likely find a PC section. Walmart & Future Shop for example. Without and investment into the used games market there's no reason *not* to stock PC games, after all they sell just as well as console games.
The point I'm getting at is that it isn't PC gaming which is changing nor has the PC suddenly begun to receive less games. Indeed according to Metacritic PC gaming was both the third largest by volume of games and the second 'best' of the platforms in 2009 by overall game rankings. It's the stores which are changing! And ya know what? As much as I love having a solid copy of all the newest games, digital sales seem to be the future for PC gaming with platforms like STEAM, Direct 2 Drive, Impulse and the like selling new games as well as smaller online distributors like GoG.com selling niche products you couldn't find in stores anyway.
And for all 2 people who read through all that, here are reference links to back up my words.
http://www.joystiq.com/2008/02/05/gamestops-used-game-sales-data-in-beautiful-chart-form/
http://features.metacritic.com/features/2009/the-best-games-of-2009/?tag=topslot;title;1
Oh, and PC gaming in Japan is weird man. It's like a whole different platform than the one I know and love. I didn't even know this obscure and strange genre existed till I took a look round Akihabara and discovered just how big those (usually porn) games for the PC are there.
