A highly compressed, global base map at 1m resolution is somewhere on the order of 10TB. MSFS is probably using higher resolution commercial imagery, and that’s just the basemap textures, most of which you’ll never see.
MSFS implements optimizations on top of that (progressive detail, compression, etc), but that’s how almost all map systems work under the hood. It’s actually an efficient way to represent real environments where you don’t have the luxury of procedural generation.
The thing is, steam’s market dominance is one of user choice rather than anticompetitive strategies or lack of alternatives. Steam doesn’t do exclusives, they don’t charge you for external sales, they don’t even prevent you from selling steam keys outside the platform, or users from launching non steam games in the client. The only real restriction is that access to steam services requires a license in the active steam account. Even valve-produced devices like the steam deck can install from other stores.
Sure, dominance is bad in an abstract theoretical way and it’d be nice if Gog, itch.io, etc were more competitive, but Steam is dominant because consumers actively choose it.