Hahha, i think we are getting a little flamewarry :)
OK, i admit, there are not a lot of great native games for Linux.
"Then why aren't games written for LINUX to run on Linux?"
Because there is no Demand for it? I really don't have a better answer for that. Linux makes up a very small percentage of the PC world, and many people run linux on sub par hardware. My thinkpad has built in graphics, and that good enough for me. I don't want my Linux OS for games.
But there are still some fun ones. Nexiuz is one that comes to mind. old school UT type game. What next, demanding why there isn't a Gaming community for Solaris?
There arn't games for Linux, because, there isn't a Market for games for Linux.
I Use Linux for work, and I used to use it for school. I still use it a lot at home for browsing/music/media related things. When I want to play games, I boot into Windows.
I Use Linux for productive things. Just like some people boot up their PS3 to play games, i see nothing wrong with booting up my Windows box.
Oh, and SDL and Allegro are no where near what you can with DirectX10...or probably 9 for that matter. The biggest problem is there isn't demand, or any developers willing to supply. I don't see the issue which makes this a Game changer for using Linux.