Linux quite obviously has a place in mainstream gaming. Playstation 3 and 4 use *BSD forks as their operating systems. The difficulty in porting was due to architecture on the PS3 (being a Cell CPU). But the PS4 is x86-*. Those are "mainstream" machines running FOSS.
To say it doesn't have a place ignores the rapid growth in just the last year alone of the number of games that have popped up which are native on Linux (not just on Steam, but across mobile devices too). But if you're not interested in Linux, you can be forgiven for not knowing about it.
Valve has done a great job in showing that Linux is simply a better platform for developing and running games on. And that FOSS like OpenGL can even outperform DirectX on Windows.