If you're a gamer, and you only own a Mac, and you haven't set it up to dual-boot into Windows, then you are not in fact a gamer. Or rather, not a particularly voracious one, as Macs are not very good for gaming at all - if you play a lot of games, you aren't using a Mac to do it[footnote]Or if you are, you've paid way too much money for your "PC", because you've just put Windows on it and are running that to play your games.[/footnote]. Not due to hardware deficiencies necessarily, the issue is there are just not that many titles released each year that are available on the Mac - the sheer overwhelming majority of all games released for a computer system are Windows titles.
Now there are ways to run games in a non-native environment (things like Wine in Linux for example) and modern Macs are based on Intel hardware, which means they're x86 based and Windows can be loaded onto them now[footnote]The older Macs used Motorola-branded RISC processors, which were incompatible with traditional PC environments, as those had adopted the more general purpose (but potentially less efficient) x86 architecture.[/footnote], so if you have a Mac and you want to play games on it, you can get around the limitations your OS places on you by just not running it, but if you aren't technically savvy (or don't have a spare Windows license around or something), gaming on the Mac is limited to the comparative handful of titles available with Mac versions.
Things like the recent release of Steam are great news for folks who, for some reason or another, find themselves saddled with a Mac but still want to actually play games on it, but the titles that are now available are still just a drop in the ocean that is all the games you can find on the PC that aren't on the Mac. Clearly there isn't any reason Macs can't run those games, should anyone take the trouble to port them to the Mac OS, but for the most part nobody does anymore; Macs are, for all the recent success Apple has enjoyed, still a very small minority of computers in use compared to Windows PCs, and the platform itself isn't perceived as a particularly viable gaming system.
Gaming enthusiasts tend to be the sort who will build their own systems and comparison shop (we find concepts like "I have the second newest model" to be ridiculous - computers are conglomerations of components, not "models"), and anyone with the technical know-how to understand the hardware that goes into a decent gaming machine will know that, for the money, Macs are a horrible choice: Apple charges a significant premium for hardware that is, once you get past their "sleek" case designs, the exact same components you could find on a shelf for a fraction of the price Apple expects you to pay. You also have far fewer options, as Apple only writes drivers for the very limited selection of hardware they offer in their products and nothing else (top-down control is a big part of Apple's philosophy; the idea that a user could put together a Mac themselves is something that gets companies who try it sued into oblivion).
So in essence, if you bought a Mac specifically with the intention to use it to play games, you've paid too much money and you're the owner of a system that doesn't actually have a whole lot of games available for it. Nobody with common sense buys a Mac to be their "gaming computer", and the people who own them now are languishing in the "here be no games" wastelands; internet wags are fond of igniting the whole "PC gaming is dying" fallacy, but Mac gaming isn't dying because it was never alive in the first place.
If you own a Mac, you're pretty much a graphic designer, a hipster, or somebody who either doesn't know any better or simply doesn't care - gamers and Macs do not really go hand in hand.
[small]Note that here I'm discussing desktops, Apple makes fairly decent laptops and a lot of the 'issues' with Mac desktop hardware are less problematic when you're in the arena of portable computing, where the options are more limited no matter which platform you adopt.[/small]
