My bet is, that licensing their world “scenery” is one of the major reasons for MSFS in the first place. As a demonstrator for Azure and Bing and other technologies. The market for 3D World mapping etc is huge.
Licensing to X-plane on the other hand, would be small potatoes in comparison, and very likely not even close to worth the cost of doing it. But I have no basis to make that claim other than looking at the potential income.
And why do people keep extolling X-plane’s flight modeling??? Sure, there’s some improvements that can be made to MSFS’s, but, it’s much more extensible than X-Planes old now technology. It’s too bad they took the comparisons of the two flight models out of the SDK.