A) why?
B) they can't. Because XP is not like some notepad app someone wrote. Rather it is a collection of software, a lot of it owned by not-microsoft and licensed to them. And that's before we talk about the security implications to existing systems still running XP.
Thirdly, what possible benefit could they get for doing this? I can see plenty of down-sides, including some very large potential economic and legal downsides. But I can't think of an upside.