Because, this:
https://www.bloomberg.com/news/articles/2017-12-20/apple-is-said-to-have-plan-to-combine-iphone-ipad-and-mac-apps
The kind of application this article talks about is the kind that will bind to Cocoa or CocoaTouch, which are similar between MacOS and iOS but not quite the same. CocoaTouch and iOS have a lot of APIs that aren’t present in Cocoa and MacOS and vice versa, becasue the set of things you do with a phone only overlaps a little with the set of things you do with a PC, and both have disjoint features.
GoW is a C# Unity app. Unity is an engine that, via C#, works on iOS, Android, Windows, and Mac. It’s not as easy as “just write your game and it works everywhere”, you do have to do some work per platform. I’m not a Unity dev, so I don’t know how much work that actually is. But “getting it to run” is just the first step, you also have to “test it” and “get it in a sellable state” and so on.
My guess is we don’t have a Mac version because the devs believe the amount of extra revenue it’d bring in won’t justify the amount of extra test effort generated. I don’t think that’s going to change until MS pulls the plug on Windows in 3-5 years like they’re writing in big block letters on the wall is going to happen.
So we could technically get a Mac version today, but there’d have to be a lot of obviously rich players demanding it.
Well, thanks for the reality check. Even though it’s not what I would like to hear. 
Relatively speaking, it’s not much; Unity is remarkably good at fulfilling its “push-button-to-deploy-to-anything” concept. But yeah, it still creates a whole new set of test cases and it’s One More Thing to maintain.