hololive

I heavily doubt it's ads, channels whose majority of the audience mainly watches livestreams usually get very little from them. And they make a TON of money from superchats (and membership!) compared to the average streamer, so I'd assume the divide is even bigger.

The main reason why abandoning Youtube as a whole might be detrimental is for the visibility. You get a lot less new users if you spin a new platform, it's a move you can afford to make only if you know your current audience is enough to be self-sustainable.