WHY/HOW DOES EVERYONE ON YOUTUBE KNOW EACH OTHER?

So I've recently gotten into the whole YouTuber thing, I mean I'm not YouTubing and making squillions off it yet, nor do I plan to, but I've been getting my YouTube watch on and what has come to my attention is thus:
EVERYONE ON YOUTUBE (who lives in the UK) KNOWS EACH OTHER!

Seriously, this is my link-by-link YouTube adventure.

The guy I lost my YouTubing virginity to (for want of a better phrase [not really, what better phrase could there be?]) was Alex Day, introduced to me by a friend of mine with his Alex Reads Twilight videos (which I uber recommend because who doesn't love taking the piss out of Twilight?). From perusing all of Alex's videos I came across Charlie McDonnel who, despite all of my new YouTube people, remains my YouTube crush 'cause he's just gorgeous. Anyway Alex and Charlie  bought a house and live together in London 'cause they know each other THAT well (platonically by the way).
Then came Dan whose videos subsequently lead me to my newest and third (second comes in later) YouTube crush but first ever twin-crush (identical by the way) Jack and Finn Harries (Finn doesn't do YouTube he just features on his brother's channel often).
Back to Alex and he brought me to Carrie Hope Fletcher who is my first, and so far, only chick YouTube subscription and I don't know how anyone could ever compare to her.
Carrie is lovely. She's just beautiful, all of them are or else I wouldn't like them so much obviously but Carrie is the one I can relate to most. She's my age, loves disney, still lives with her parents and does normal stuff. Her world hasn't exploded because of the YouTube thing, or she hasn't let it.
From Carrie I discovered the duo Jack Howard and Dean Dobbs and from there Jack's solo channel. Jack Howard (Jack the First as I call him - as a contrast to Jack Harries = Jack II) is stunning. He looks like Jonathan Rhys Meyers and he's quite witty too.

The point is, Alex knows Charlie, knows Carrie, knows Dan, Carrie knows Jack I and Dean and Dan then knows Finn and Jack I.
HOW DOES EVERYONE ON YOUTUBE KNOW EACH OTHER?
I get that like they go to YouTube conventions and VidCon and stuff and also watch each others videos   and tweet each other and stuff but it just BLOWS MY MIND that they just feature in each others videos randomly. In a cool way though. I mean, if they hadn't I'd still just be watching Alex Day.