Okay this is my two cents:
1. Larger YouTubers (100,000 subs+), whilst they need to work on their networking, don't need it as much as a newcomer or a long-term smaller channel. So they are less likely to sign up to a forum to aid themselves.
2. Larger YouTubers (save but a rare few) don't feel the need to mingle with smaller channel people. We have a few members here who are big (we have iJustine, for one) but she has never posted and likely never will. We've had one or two larger 'Tubers who posted once and never came back.
2a. The above has nothing to do with the fact that they are off working on their channels. It's just common knowledge that larger YouTubers can't be bothered to mingle (or even associate/speak with) smaller ones, which leaves all the smaller ones in a group on the forum.
3. If you think it's pointless, then why are you here? I don't say that to argue, but seriously. This forum's not a promo forum (it has a promotional privilege offered to those who are active enough, and I speak from experience in the past that forum activity = views and subs). If you go to a forum expecting to grow people have to know you. Sure that's a bit clique-y but hey, that's life. People also have to like you. So if you're here to promote and grow only, then perhaps you're better off with reddit or facebook etc.
4. ALL of the larger Tubers (10k and above) were large before they got here. They're not large because they're here. And those here who have grown quickly have done so because of external reasons like famous Tubers have retweeted their link, or featured them or whatever.