Agreed. You have to remember everything is drive by economics in our capitalistic society so games are written for the OS that has largest market penetration and most likely to return huge profit and currently that is Windows. That is also why one of the most famous game programmers, John Carmack, just recently rewrote some of his games for iPhone/ipod touch and made quite a nice profit from his own account.
"Carmack said that there was a lot of "hand wringing" initially over the idea of spending the company's own money (there was no publisher to help fund development) on making games for the iPhone. Doom Resurrection, when it hit, was probably the most expensive game to develop for the iPhone, Carmack says.
"It did really well for us," he said. "It was Wolfenstein Classic that made the argument for iPhone development for me. We made quite a bit of money off of that."
http://kotaku.com/5397350/carmack-on-iphone-fallout-quake-live-and-elves-and-orc s
That is not to say that Mac's can't be used as game machines since Apple actually has a webpage dedicated exactly to this you can check out:
http://www.apple.com/games/hardware/
So in a nutshell to answer the OP question the reason you see fewer games on mac's is because it's not profitable. As you can see if there is no technical limitations writing games like Doom3 for iphone there is none for the mac.
I know I've personally not bought a macosx game since I also have a pc and use that for games so I'm not helping the business case for more games for mac. Vote with your pocket and go out and buy more mac only games if you want to see more for the mac.