I've talked about sponsorship with some big YouTubers and they said it's not worth the effort. It's limiting themselves with contractual terms and conditions and even goals they have to reach in order to keep the contract etc... And most of all - you lose your freedom (i.e. sponsorship is like getting hired by someone to do a job - most of how the commercial world works anyway). People who are purely for the money in YouTube would love sponsorship. I myself prefer no obligations, and I film and review what I see and what I like. Finding products to film/review is a hard job without sponsorship though. But I find friends/relatives that buy stuff and lend it to me, so I always have what to review.
+1 Yes, OP seems to be talking about partnerships with MCN networks. Companies who out ads on your channel and you have to pay them a percentage of earnings.
"Sponsorships" are very different. They are when companies pay channels with money or in kind (with free products and services) and in return, the channel creator gives them publicity in their videos. (product reviews, shoutouts etc)
(I've moved this to the monetization / partnership forum)