Huh? You're on their website to watch videos. And it costs them money to send you those bits. And they offer two ways for you to compensate. Watch ads, or pay premium.
What is so difficult for you to understand this business relationship?
It's Google. The relationship is not consensual but adversarial. Google attempts to get free things from me. I attempt to get free things from Google.
It's like asking a lawyer why does he defend an obviously guilty client? Because it's adversarial system, his job is to protect his client, not to worry about the other side. The other side is trying to maximize their advantage too. Google has defined my relationship with it in such terms through its behavior.
If YouTube were still an independent operator I would be more amenable to your argument.
In any case, the fact I can recite an ad from memory shows that I am at least watching some of their ads, notably on mobile.
What is so difficult for you to understand this business relationship?