Probably because the installer is pointing to the wrong folder. This has come up a few times, but there’s been no explanation why some see this, and not others.
I’ve never had this issue myself, and the installer has always pointed to the correct folder. For me that is “D:\Flight Simulator”, and it contains the following two folders:
D:\FLIGHT SIMULATOR
├───Community
├───Official
As long as the installer points to the folder containing those two folders it should be fine.