I am by no means an expert in game development, but a quick google search seems to contradict your claims regarding Unreal Engine:
http://www.unrealengine.com/en/platforms/
(aside from the fact that UnrealEngine 3 is stated as fully supporting both Mac OS X and iOS - in fact, those are the very first two platforms listed in their website -, UnrealEngine 4, which seems to be in its very very early stages, states compatibility with "PC, PlayStation 4, and Xbox One, with
other platforms to follow"...)
Also, regarding CryEngine, an OS X port is something that Crytek clearly don't underestimate:
http://mygaming.co.za/news/pc/56362-cryengine-coming-to-linux.html
and
http://www.crytek.com/career/offers/overview/frankfurt/programming-engineering/junior-programmer
Dice, the company behind the Frostbite game engine (which powers the acclaimed Battlefield series) also seems extremely interested in OS X gamming:
http://www.groths.org/frostbite-game-engine-coming-to-mac-osx/
Electronic Arts which, as you surely know, is one of the most important game developers / distributors recently ported their Steam competitor (Origin) to OS X and started distributing OS X versions of many of their games:
http://store2.origin.com/store/eaemea/pt_PT/cat/mac/categoryID.62022900
Also, there are other Engines out there you know? And some of them support OS X just fine...
http://unigine.com/products/unigine/licensing/, for example...
Another major example, the Source Engine, seems to be very much into OpenGL, and consequently into OS X and Linux as well... from
the Wikipedia article:
So, while OS X can represent only 8% of the PC Market (taking your word for it), OS X + iOS + Linux Distros + Google's Android and Chrome OSes + every single gaming console out there except for the Xboxes, really start to constitute a market one should consider when developing games (and game engines for that matter).
If the engines support OpenGL, porting PS or iOS or Nintendo-ready engines to OS X (and Linux and Android and etc, etc...) should be fairly easy and would probably make economic sense in most of the cases.
Also, keep in mind that OpenGL is platform agnostic (both software and hardware-wise), so it only makes sense that game (and game engines) developers start to take into consideration the advantages of having their games (and game engines) ported to OS X, Linux and the likes...
Just my 0.02$ though