Much as modern dance music is made up of samples, and postmodern art made up of “found” objects, re-assembled by the artist, Tumblr is about using the found to say something different.
Curation is the new kind of creativity. You don’t necessarily have to create original content to be recognized as a creative type. You just have to be excellent at remixing disparate things into holistic stories.
This is what DJs do, after all.