That's where the problem is, a lot of popular content creators have contracts with large media groups which specifically prevents them to upload to another platform.
Do the media partners in turn have contracts with youtube requiring that they upload to it exclusively?
If not, maybe even if so, that might not be such a bad thing. It would mean that if you could negotiate a deal with a single media group to upload to your platform as well you get a lot of content. I don't imagine these businesses are any happier with being hostage by youtube than individual creators.
That'd work fine for small channels, but you can bet that the big ones do have exclusivity contracts. Not with YouTube directly, but media groups that tie them to YouTube.
In-video sponsors like Skillshare, NordVPN, Dollar Shave Club or whatever don't contact channels directly. There's someone representing them.