This is because these collaborations don't happen in one "place" on the
internet, they are started somewhere, added to somewhere else, and are
collected somewhere else. Someone puts an idea out there, usually
here on this list and on their blog, and others just start "doing".
As for rules, Google told me:
On another note, I'd like to see more vloggers doing regular
collaborations, like the wearethemedia people. Themed vlogs/blogs
with several contributors. Like, I want to contribute to a
science vlog. I'd like to see a collaborative sports vlog.