If you want groceries you have to pay. If you want YouTube videos you need to pay by playing the ad(legally speaking, obviously you can steal if you like). I don’t see any difference.

Where's the obligation to watch ads spelled out? The legal obligation to pay for groceries is spelled out in the law: they are the possession of the store, and if you want to acquire them you need to exchange something else of value (money) for them, at which point they become yours.

What is the thing that compels you to watch ads on a service like Youtube? There's nothing in the law; if there is anything, it would be spelled out in the Youtube terms of service: https://www.youtube.com/t/terms

Can you find it for me? I've looked. Many times. It isn't there.