Pewdiepie and shaycarl don't really promote lol. By having such a big fan base people share the videos, and that's why they grow, not because of connections.
"I don't know many big YouTubers that simply put a video on YouTube and do nothing else." ... If you look at pewdiepie, that's pretty much what he does... He's big enough to do that, and I know many people who are at the 100k+ sub level that do the same thing. You don't necessarily need to promote when your big because your subscribers will promote for you essentially.