For the same reason Ubuntu doesn't just ship the kernel and expect users to download bash themselves. Users *expect* that when they buy an "operating system", it will come with certain key components. These days, for desktops, that includes a graphical interface (X) and a desktop (Gnome &c), along with a web browser and multimedia applications.
Re the "middleman", I definitely feel like the "middleman" is a feature. Look at all the crap that Windows users end up installing because there is no middleman! Because there's no one vetting the software and making sure that it's up to snuff, each individual piece of software wants to at least install its own update-checker and its own useless piece of junk in your status bar, if not its own spyware. With Ubuntu, they can have a reasonable policy across the board for that stuff, and as a result, my Ubuntu desktop is always more clean and usable than my Windows desktop. Even with the current "spyware" issue, I trust Ubuntu a whole lot more than I trust all those random "application providers" that might want to install who-knows-what on my system.
As someone else has already pointed out, Google and Mozilla both know everything that Canonical would know anyway. And after taking some business classes, I can sympathize with wanting to understand what your users are doing so that you can serve them better. (Asking is of limited use, because only certain kinds of people answer, and even when they answer, it may not be representative.)
My main concern is that new versions of Ubuntu not end up like a new Dell box, packed full of junk that I have to un-install before it's remotely useable. That's one of the things that I *really* appreciate about Linux distros: not having to deal with random junk.