It's because Windows uses the concept of subsystems for lots of pieces of its architecture. If you look at a process listing, you'll see lots of processes named "*ss.exe", where the "ss" stands for subsystem, like lsass, csrss, smss, etc.

Also back in the day Windows supported POSIX as a subsystem and some others as well. Each one was considered a Windows subsystem for that specific architecture. And now that there's one for Linux, we get WSL.