If you separate things out, you have to now manage version compatibility among different components. Which is a massive waste of time when they are going to be

If you separate things out, you have to now manage version compatibility among different components. Which is a massive waste of time when they are going to be shipped as a single unit anyway. Things are indeed being broken up into packages, but they still all live in the same repository. Microsoft goes one step further and checks the entire build system into the source tree too, compiler and all. The effect of all this is that you can sync to any known-good version of Windows and just build it without worrying about other dependencies.