iTunes isn't built on Cocoa: http://daringfireball.net/2009/09/itunes_and_cocoa

And that's why iTunes is so bad - because it shares code with the Windows version that can't be Cocoa.

Lots of people write great code on Windows. There's no reason why Apple couldn't too, if it cared.

I agree they could make the core cross-platform and build platform-specific GUIs, but, apart from making Windows appear to suck less when compared to Macs, what would they gain from it?

I don't particularly like iTunes, but it does two jobs for Apple: it syncs iPods and makes Windows look bad.