Because the Mac OS is a different platform than PC. If you look at retail Mac games you'll notice two things. One, they usually cost more than their PC counterparts. Two, they usually don't drop in price the way PC games do in a relatively short amount of time. Usually, they stay at $50 or whatever the original MSRP was.
While I'm not sure how they handle game development for the Mac OS, I believe Apple does receive some sort of royalty fee for games sold for their platform. It was like that in the past, don't know about now.
When you pay for licensed software, usually, but not always, it's for the specific platform you purchased it for.
All Adobe software for instance is platform specific. You pay for Photoshop CS, you either buy Windows or Mac. You don't get a license for both. I wish you did since I bought the Production Premium bundle for Windows and would like to also install it on my MBP, but you don't get a free lunch from Adobe.
Autodesk is one creative content software company that does offer dual licenses on many of its applications like Maya, but this is very generous IMO and not a given that they have to do this.
Now if Valve wants to be very generous and give its customers free licenses for the Mac counterparts of games they already have in their PC lists; great. All Hail Valve. I'm just not expecting it because when you buy a PC version of a game and you want to play it on a Mac, you either have to buy the Mac version of the game, or play it through Windows using Boot Camp or any of the other utilities that allow you to run Windows on a Mac (a lot of PC users seem to gloss over this feature).