Promoting a video or channel costs money. Unless a channel is earning hundreds to thousands of dollars a month for a network, it's simply not worth them promoting it.
At best they'll put a segment of a channel's videos in one of the networks promotional videos, but, considering the vast majority of people who view those are people within the network looking for their own video short, they do very little to actually boost a channel.
Being worried about copyright and wanting another layer of safety between yourself and AdSense or wanting your videos part of ContentID (if 100% ALL content is yours) are about the only reason to join a network nowadays.
They won't get you better, higher paying ads, they won't do anything for your views, and the vast majority of features and options they can give you (need it be music, a better dashboard, or a community) can be had without signing away 10-40% of your income.