Macs are not gaming machines. They never have been. There was a time maybe 10-15 years ago when you could build a mid-tier gaming Mac, at significantly higher expense than a PC, but Macs have never catered to anything more than very casual gamers. They don't run other games "easily" - they just adequately run some games, barely run others, and simply can't run a lot of popular titles (if they exist at all).
Every Mac, with two exceptions, uses integrated graphics on their Intel chips.
The MacBookPro can optionally can be fitted with a Radeon R9 370X, and the MacPro has the FirePro option above. Let's look at 3D performance of these various options (The FirePro D700 is a Mac special - it's roughly in line with the specs of the W9000 PC variant):
The multi-thousand dollar FirePro is sitting there next to an old GTX 670 that you can pick up in used bargain bins these days. So what gives?
The FirePro is a professional GPU - it competes with NVidia's Quadro and Tesla line of cards. It is designed specifically for precision workstation GPU tasks, particularly for double-precision calculations. These are important for CAD/Architechtural and Engineering applications as well as for scientific computing. The cards can be used just fine to play 3D games, but they are not optimized for that task and naturally underperform when compared to high end gaming GPUs like the GTX970/980, etc.
Gaming GPUs dedicate resources to lower precision calculations (which they can do more of, and more quickly) and eye-candy goodies. The tasks which they set themselves to are largely at odds with each other in terms of the style of hardware that is needed to get the job done.
If you wanted to build a gaming mac right now, you really can't. None of the systems available even have slots anymore to add in aftermarket GPUs (and they were twice the price of PC GPUs back in the days when you could, and came out months and months later than their PC counterparts so were always behind the cutting edge). The only expandable Mac right now is the MacPro, and it is so ludicrously expensive that you would be crazy to try to make a gaming machine out of it (in fact, the hardware simply can't be had). With expensive Xeon processors and FirePro graphics, the system is a highly specialized workstation that is firmly aimed at creative, scientific, and engineering professionals.
So where does that leave the Mac? Absolutely, completely out of the gaming arena - full stop. It's got enough juice to run a few very casual games at very modest quality for home users, but that's just about it. The Oculus Rift needs major GPU horsepower. For 3D it needs to compute every scene from two unique perspectives (one for each eye), instantly doubling the amount of pre-rendering calculation it needs to do. It then needs to render each of these independent scenes to two independent displays and keep everything moving fast and smooth enough to not make you want to vomit or have some sort of seizure. That means it needs a powerful gaming GPU, and these just aren't available for the Mac anywhere.