Why do so many games not have Mac versions?

I'm not a game designer, nor am I much more than a novice programmer.  In the past, I always assumed the reason games don't get ported to one environment or another was because the architecture of the systems differed so greatly that you'd need to be reasonably assured there was a market on other systems.  But just about everything (Playstation, Xbox, Mac, PC) uses x86 architecture now, doesn't it?  Or do I really just not know what I'm talking about?

But assuming I'm not wrong with the above assumption, then is the problem largely just the different OS?  I would think that the dramatic variability in how PCs are built from player to player would make things a lot harder than making a build for Mac or PS4, for example, so is there something about Mac OS that makes it unappealing to game developers?

Anyway, I'm just asking purely out of curiosity.  I'm not secretly asking for a Mac version of SCO, just curious why virtually any game doesn't come out on most platforms in almost every case?  (Best guess: I just haven't got a clue how things really work!)