They can't. It would require completely different technologies.
If you data set is render-only, you can accomplish this using satellite imagery, with some AI-processing for eye-candy.
For military stuff you need modifiable environment and a well-structured data set for the game logic to work.
One can only imagine an amount of work to get and support such data set on the Earth's scale.
Btw, Bing has quite a lot of structured data like roads, parks, lakes, buildings... We'll see to which extent they actually use it in FS 2020.
Having a map of roads + satellite images and having a believable realtime rendering of these roads embedded into a terrain are two different things.