My personal theory: Youtube ads exist as just a punishment to beat you into paying for premium. As in, while the ads make some money, their primary purpose is to motivate you to pay. And that a subscriber generates meaningful more revenue than the ads sales of a non-subscriber.

It seems plausible to me, since I get so many low quality poorly targeted ads on repeat. I can't imagine those ads are generating much revenue, but it makes sense if their primary purpose is just to beat me into paying for a subscription.