The simplest answer is: iTunes does not sound good.
With all the other options for better sound output, that still use the iTunes back end, I am not sure the simplest answer is the best. Any of Amarra, Audirvana, Fidelia and others can produce better sound of an ordinary Mac, while still using iTunes for one's organizational tool. I am not an iTunes fanboy, though up until some of the recent UI changes, I was happier with it. And I still use it for organizing, tagging, etc. I don't rip with it, but—knock on wood—it has been a fine, stable container for my music [about 15,000 tracks].
Indeed, even a modest setting in CoreAudio can rectify most of iTunes' sonic imperfections. For example, using iTunes only, and setting CoreAudio correctly, can deliver quite fine sound through a PWD. Last time I checked though, that's still limited to 96/24. But still quite good, and in fact, with the right player, even lowly 44/16 can sound fine. Depending on one's DAC, UPnP servers can also deliver quality sound. Of course, if one is delivering the analog sound out of the Mac, neither iTunes nor MC will make much of that mess, though I assume you are referring to digital output.
MC is a significant development project. Sound alone would not justify that; that market already has good contenders. Obviously, it offers the potential of a great deal more, and it could become quite the alternative to iTunes, though like of foundational integration with the iTunes store will always mean it does not sleep in the master bedroom. But for me, I am simply looking for the best, most robust UPnP server/client combo. EMM is not bad, but it is not up to the standards of the $4k DAC for which PS Audio supplies it. Even in this pre-alpha state, MC is already faster than EMM. Whether it will be more robust—and my bet is that it will—remains to be proven, but I am enthusiastic about this project.