Except that if you start a new tree for any reason (and when doing integration work, this is routine) the only way to bootstrap it is with a full build of a 9+GB tree. Once built, you have some options (though frankly they're pretty poor in comparison to an RPM). But there's no escaping the huge bootstrap, and it's an immense pain that simply doesn't exist in the package management world.
Contrast this to a desktop distro, where if I want to cook up a special version of some package or another I can just do it in isolation (obviously with some caveats: changing runtime compatibility still requires full dependency builds, etc...). At no point does Fedora make make me build the whole world just to install an image with a new kernel or X server. Things like glibc and coreutils and gnome are already built (and signed!) and ready for use via a trivial download.