Manufacturer launchers?

I've always wondered why the manufacturer's skins were not just supplied as a launcher on top of vanilla Android?

Obviously some manufacturer customisations go deeper than others but this way base Android could be updated independently of the "enhancements" and most additional functionality/widgets could be replicated by pre-installed apps. Preferably uninstallable of course.

Widescreen

Can't help thinking this UI would work better on tablets with a 16:9/16:10 aspect ratio...