Honestly, even if they did release the source, I'm not sure what good it would do.They'd probably have to hire authors and train them so that they could write books solely documenting the various APIs for anyone to be able to even make minor modifications.
Remember the Win2K source leak? I believe that's almost a decade ago. And back then, Microsoft claimed the entirety of their kernel and OS-related sourcecode totaled 4 terabyte.
That was back then.
Try to imagine several decades worth of backward compatibility fixes, third party vendor support, custom build scripts, backwards compatibility support for drivers (we're talking really low level code here), security fixes, and building on code that should have been rewritten but wasn't because there wasn't enough time or money or because said code fixed some really asinine bug that anyone has yet to comprehend.
That's the Windows code base in a nutshell.