Xbox is a different strategy, it's about content and subscription revenue streams. Releasing a Mac OS compatible Windows "app" really doesn't support ongoing subscription sales nor would such a market be larger enough, IMHO, for MS to justify making and supporting it. Windows has a large OEM market base so ongoing development means ongoing sales, and that allows MS to offer free upgrades; plus it means a more up to date OS base for MS' products as well.I’d believe that if Xbox wasn’t a thing.
Once MS sold into the market for an Mac OS Windows product, the growth would be small, so either they charge regular update fees, like Parallels, or give it away like Fusion, neither of which is worth the trouble for such a small return.