If they did that they lose all of the benefit of Stardock. If Start8 was in in Windows8 it would need to localized, made accessible for users with disabilities, maintained, continuously upgraded and supported for enterprise for 10 years, all for the tiny numbers of Windows8 users who actually want it. It would also either be on by default, effectively defeating the entire point of apps-front-and-centre philosophy of Win8, or it would be off by default, and we'd be right back here. And let's not forget that start8 is in total contrast to MS UX guidelines, so if MS did buy it it would be impossible to rectify MS policy with having stardock, and ultimately that would mean dismantling stardock and moving their staff to work on UX for other teams at MS, i.e. killing start8 stone dead.
Also, what's the scenario that users want that they don't currently have? If you want a start menu you can get one. From Microsoft's POV that means that there is now nobody who can't upgrade to Win8 for lack of a start menu since everyone either doesn't care or can get one from StarDock.
From Microsoft's POV the best possible thing is for someone else to make the apps for transition between UXes. Microsoft wants to support the old one and the new one. It doesn't want to support the infinite different possibilities between the two, and as time and time again they've found out, adding configuration options and extra knobs and dials and buttons in the interface leads to extra confusion and extra problems.
No, the best possible solution for Microsoft is that someone else sells the apps that help smooth the transition between Microsoft's own products. Microsoft seriously frikken loves Start8 because it destroys the argument that people can't upgrade to Win8 because Win8 is too unusable without a start menu; because Windows8 has a start menu for the tiny numbers of Win8 users who really want it, without needing to support it for everyone else.