I think a big factor is that it's low friction. Just open the link or search whatever you want and it plays. It's not like cable where you need to sign up for a service, or Netflix where you need to scroll around in previews selecting for your next show, it's always on your phone, laptop or TV fast and free.
It's successful because it's mindless, people can just pull something up and consume content. If they start pushing more unskippable ads, or requiring subscriptions or accounts to view, their viewership would go way down and people would move on to next easier thing.