Youtube is ad-supported. You can't upload videos more than 15 minutes long without letting Youtube put preroll and mid-roll ads all over them, and the only access-control method it supports is "unlisted", where anybody with the URL can see it.

Creators don't want to force their paying Patreon supporters to watch ads for content that they paid for, and that can be leaked to the whole world if somebody merely ctrl-vs a bit of text.