Because "URI" stands for "Uniform Resource Identifier" (or URL for "Locator"), not "Resource". The intent is for it to be a pointer, not the value at the location. And, if you were to use the URI as the content (instead of chunking and shrinking it via Bitly), you'd be duplicating that content on every page that links to it. And in your browsing history, by merely viewing it.
edit: oooh, another thought: you're essentially uploading the content of the page to view it.
There's an interesting copyright question in there somewhere too. If the URL for my document is the document then sharing the link is infringing my copyright, or something.
there is such a thing as fair use in copyright law (at least, in australia and US and most major western states).
I don't think there's any abuse here (after all, data: URIs exist). A value is just another kind of pointer.
The important thing is that the identified resource is unambiguously identified.