Yeah, every YT creator that is serious about their job should have their own website with a copy of the videos, and I find it really curious that this doesn't seem to be much of a thing? At best I'm seeing merchandise webshops. But you'd think these people would be multi-channel and have a website, youtube, all the social medias, etc, and the bigger ones a company to manage them all.
But I suspect that as they get bigger, they enter in exclusivity / no-compete contracts with Youtube, and if they detect the same video hosted elsewhere, they get taken down or something.
This sounds like an opportunity for a product. Apart from eyeballs and familiarity, Youtube does a lot of handholding so that non-technical people van run their own channels. I don't think 90% of youtubers would have any idea how to spin up a website. But I'm sure they'd be happy to pay someone to do it for them (as long as the price was a small fractuon of their ad revenue).