because for making it easier to provide tech support and mangage the current versions, shipping a complete and known package is easier. randomly downloading different parts as and when just isn't helpful for a software of this size (fine if you're messing around with an internet browser). also it's easier for the devs to maintain a single complete install.
and it's not like this hasn't been talked about before but until a real need and then the required infrastructure was put into place, how it's distributed is unlikely to change from how things have been done for however many years. and so what if it's 2006, what next, the whole of the winamp ui should just be a tabbed interface? somethings just aren't appropriate across the board at the end of the day.