Really not gaming computers? In terms of graphics, ability to run and perform tasks needed for the higher end settings of a game? Or more like no company has made a good effort to produce a OS native game that requires such abilities.
In terms of the raw 3D power of graphic cards. Macs are generally equipped with mid-range graphic cards as best. The exception is the current Mac Pro with a Radeon 5870 which is pretty powerful, but even that card was released about 1.5 years ago and there are noticeable more powerful cards on the market now that the PC gamers can buy.
Also, not long ago some developers, including Valve, used to complain about how slow and poor the OpenGL drivers by Apple were (for some reason Apple decide to write all their graphic drivers instead of letting the more experienced graphic card manufacturers deal with it like on Windows). And there are plenty of benchmarks around the net showing that Windows and Linux offer
much better OpenGL performance on the same hardware, even on Macs running Windows! So when ported Mac games run worse on Macs than on Windows, it's not really the developers fault, but Apples fault.
So there are two issues, the lack of raw 3D power from the hardware and the lack of Apple making good effort to produce a fast native OpenGL driver. There is also a 3rd issue that people generally don't see Macs as gaming machines, so people don't bother to buy games for it. Apple never really cared about games much, the gaming success on iPhone pretty much caught them with surprise. So now they have focused on making the iPhone/iPad more gaming friendly with good graphic APIs and stuff, but still kinda ignored the Macs.
That said, usually mid-range cards are often enough as long you don't play in high resolutions or with lots of AA though and Brink is a game also developed for 360 and PS3, and that's old hardware now, so it would probably run rather well on Macs.