Earlier quoted context omitted.
The main reason these days is that Macs don't have good GPUs for games.
That's not true. Even current Macs with IGPs can play nearly any game at low settings adequately.
I tested this with Quake III (a game from 1999!). My 2011 iMac could not render the game at 120fps, it managed somewhere between 50 and 70. I bootcamp into Windows, and I get a rock solid 120fps.