Your Mac
is a PC. You paid a premium to be able to run OSX legally, not for a different machine. When you buy a Mac all you are doing is buying a seriously expensive OS.
More on topic with the OP:
According to Valve and nVidia's presentation in GDC2013 (pdf:
https://developer.nvidia.com/sites/d...to%20Linux.pdf / video:
http://www.gdcvault.com/play/1017850/ ) Linux gaming is on the rise and Mac game is either stale or slightly falling.
These platforms have a very small current market share when it comes to video games, but it's very expected that they will rise in the future further.
The main problem that current developers face is their original decision of graphics API, much more than OS platform. I'm referring to the usage of DirectX instead of OpenGL, which is the only option for both Mac and nix platforms.
I won't go into any detail about the matter, but the point is this isn't trivial.