I'm guessing that would be a licensing nightmare. They must pay many companies for licensed technologies inside Windows, and many of those licenses likely wouldn't be compatible with open source licensing.
All of their source code would have to go through legal review, some with each check in. I don't see that happening for legacy code.
If you look at how long it took for Sun to make Solaris free software (and even then it wasn't truly free in some cases) I doubt Microsoft would ever consider spending that much time doing it.
It says something that MS chose Git over anything proprietary that they developed.