The X-Plane simulator is Certified by FAA!!! and they sold less copies of it than FS series.
So why MS don't certify his simulator? Simple the flight model and the Engine itself is a total mess!
In FS you can fly a 747 with cessna wings! in Xplane it take account of Airfoil and simulate fluid (air) dinamycs from them!
.....
But i think as a procedure only trainer MS can be certified....thats are only my toughts.
Bye!