Because the competition is already fierce. There's MS Flight Simulator and X-Plane on the commercial side, Flightgear on the open source side and geo-fs.com on the free-to-play side.
There is not much Google can actually gain from making their own flight simulator.
Grand Theft Auto is now doing it, but Google Earth would make more sense because it can bring a more realistic environment.
https://www.reddit.com/r/technology/comments/1szcl5i/google_...