It's not bug fixes.
Since Apple want iPods to be windows and mac compatible, they make iTunes cross platform too.
iTunes contains all the code for handling new features in iPods, as well as itself. So when Apple bring out a new iPod with new features, they need a new iTunes version to support those features.
The recent versions are due to the added ringtone support etc. in iPhone (also uses iTunes).
Mac OS X has got a good syncing framework that should be handling this, but to save having to implement that under windows too they just added this stuff to iTunes. Annoying, but easier and faster to make it cross platform.