Because the game already also runs on Xbox and, given MS's recent gaming strategy (which is putting less emphasis on Xbox exclusives), could conceivably come to Playstation or maybe even Switch 2 in the future.

On the Windows side of things, there's also a push towards ARM hardware (with current Snapdragon-based hardware actually performing pretty well). Not sure if Flight Simulator is currently ARM-native, but having the ability to go ARM-native is probably desirable at least as a long-term goal.