gnu/linux is not designed for games, any attempts to add proper support has ended short; steam for linux is the closest thing you have right now, or games from the 90sif you want performance out of linux, you'd want a minimal distro, but that still wouldn't offer gaming performance
Games can be coded in OpenGL and would therefore be available for every operating system, excluding ones such as 'Raspberry Pi'. It would experience a backtrack in time, and you'd probably be staring at GTA Vice City graphics for most low-cost games. And the only reason it's not changed so far, is that basically all the mainstream Graphics Cards are built to support Directx.
