the time in which the TV is on but users aren’t doing anything is valuable

Are they going to pay for the increased power to do so?

Ads have funded a lot of content in the past. I don't mean just in the Internet era, but in the TV era and the radio era and the newspaper era. We're talking centuries.

Unless you're gonna get people to pay for your content, which can create difficulties, attaching it to ads can be a way to pay for that content.

Now, all that being said, that isn't to say that one needs to want to choose ads or needs to want to choose ads in all contexts or can want unlimited ads. I'd generally rather pay for something up front. Let's say that it takes $10 to produce a piece of content. For ads to make sense, it has to make the average user ultimately spend at least $10 more on some advertised product than they otherwise would have, or it wouldn't make sense for the advertiser to give the content creator $10. I'd just as soon spend $10 on the content directly instead and not watch the ads. Ultimately, the average user has to pay at least as much under an ad regime as if they just paid for the content up front, and doesn't have to deal with the overhead of me staring at ads.

But for that to work, the content provider has to be able to actually get people to pay for whatever content they're putting out. If it gets pirated, or people disproportionately weight the cost of that up-front payment, or people are worried about the security of their transaction, or what-have-you, then the content provider is gonna fall back to being paid in ads.