Because they're worse alternatives, not to mention the MONEY people make on Youtube. RSBN has been making serious cash in ad revenue, subscriptions and super-chat donations through Youtube. Not to mention the ease of use of the platform when you create content and upload it. It's easy and fast to upload, you can schedule uploads, you get all kinds of information about how your uploads are doing. Then there's the reality that so many people use this platform and Google is good at getting eyes to your videos.

So going to a new platform generally means NO money making ability, MUCH less people, a worse user experience (for the uploader and viewer) and MUCH worse marketing. Google is damn good at what they do, love them or hate them that is undeniable. It goes far beyond just needing a website to throw a video onto.