That is basically the lack of development of OpenGL itself.
The condition of Mac in gaming (i consider palying AAA games, because you can play other games quite well on Macs), the only to blame is Apple engineers. Even that they consider not to blame, since it wasn't a priority for them.
Go no further then Metal, which has been announce more then 6 months ago and we still have no further news aside what was announced on Apple Keynotes and even one producer cough"Adobe"cough, even backtrack on their development on Metal. There isn't even a proper comparison between OpenGL and Metal, we haven't seen the improvements Metal would bring. Blizzard seems to be the one that is actually will be releasing some stuff in development of WOW to Metal.
Even Apple has made little to no effort at all, to marketing how amazing and spectacular Metal is. Apple on the laptops and Desktops have no interest in games, for some reason. If it was, they would have make effort at least to push for Metal.
As in terms os PCs, yes it is less expensive and game oriented then the Mac, but one thing I myself notice, is that I spent more on PC in timeframe of 5 years then a newly IMac 27", for example. Sure my focus was purely gaming, but as I grow older, I have little pacience in building a PC and due to lack of time, and my focus changed to those quite essential games. The majority exists on the PS4 or XBOX ONE, which is a lot less expensive option then a PC or a Mac and lasts quite a few years. The rest exists on the Mac and can run quite well.
However there is important point. Sure a PC is a lot less expensive then a Mac, but a friend of mine bought a PC and build is PC, in August 2014, last month we decided to see what would cost him the exact same PC, it it would cost him half the price he spent in 2014. I was quite suprise how it devalued so much so fast. the mac is expensive and devalues a lot less in the same time period.
Which brings me to the last point of this post, I find a bit of disgust by Apple still charging the exact same price for the Mac Pro that was released in 2013. I for one think Apple needs to adjust the price as time goes on, or simply update their desktops and Laptops more closely. They don't need to be revulutinary in every release, but be serious how can I buy a Mac Pro from 2013 specs for that kind of money? 3000 dollars? It is mindblowing and shockingly bad, in my opinion.
In the end, Mac and PCs have some pros and cons, but Gaming for Mac, the PC is clearly a winner, due to lack of interest of Apple and its engineers and head of department to push for AAA gaming for the desktop and laptop lineups. I think Apple is apparently happy to bring those IOS games to OS X, since both using Metal, the ports I think would be quite easy and expands the market with Apple's Market. Apple is always question mark, but I kinda like that it creates these colorful and intriguing discussions, bringing the Mac community together.