There's no default oven or toaster or blender either. Because you buy the product to perform a function. If you buy a TV, you want to receive TV signals, or play content on a device that plugs into a TV.
If you have a computer, it can do damn near anything so there needs to be defaults for the things it can do, to expres to new users that it can do them. If every computer came with a default feedreader pre-populated with a set of feeds and explained "hey, click the [icon] on your favorite websites, and we'll show you new updates right here, like a customized newspaper" there'd be larger uptake.
People buy toasters not because there is a default one but because they've used one in the past and got used to the idea. We need to focus on getting the idea of possibility of RSS-like functionality across, not on a nitpick like lack of a "default" client.
It would be an interesting twist to see RSS explained as "Twitter for any website."
But TV is something that people already know that they want, so they're more willing to jump through hoops; they'll fiddle with it until it works, or call up the cable company and say "make it play HBO and sports". RSS, on the other hand, is something people won't want until they've used it and integrated it into their lives. This is a microcosm of adoption of the internet itself, which only took off once (a) browsers were part of the OS, and (b) AOL disks were everywhere, combined with social forces turning it into a de facto "default client".