From there, you also ensure that you have a backup of all your videos. I've talked to people that only had their stuff on YouTube/Facebook/whatever. It is super risky. If you have a backup, and YouTube bans you, you can rehost elsewhere, it won't be as big, but you might still have a business afterwards.
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.