Just HOW MANY & which games ARE pure 64-bit nowadays?
There is a reason why most games are still 32-bit apps.
The big advantage of 64-bit instructions is that they can handle large amounts of RAM. If you aren't using a ton of RAM, there's little benefit to switching instruction sets. Until recently, most high-budget games were targeted at consoles with tiny amounts of RAM.
Even today, brand new computers are shipping with 4GB ram. I'm not just talking about Surface Pros and Macbook airs... Alienware is selling a dedicated gaming PC with only 4GB.
PC game developers know that requiring more than 4GB would sacrifice a chunk of their audience. So why bother porting to 64-bit? They can't really take advantage until all those 4GB machines go away.
Things are starting to turn around, though. Sony and MS have finally released consoles with 8GB ram, so we should expect to see 64-bit games appearing in the form of console ports.