The .NET framework library source code is available. If you are a national government or very important partner, you can take a look at what is supposed to be much of the Windows source code, but you can't build and run it.

Microsoft derives a lot of income from enterprise licensing and support, so it's not obvious just how much income Microsoft could lose by going open source.

Windows CE had to be abandoned for mobile devices because it had fallen behind Linux-based mobile device OSs. They probably continue to believe Windows is superior to Linux, so they, internally anyway, have a "secret sauce" argument against open source for that.

Perhaps they will have to go truly open source to regain trust after the Prism scandal.