There's plenty of content that advertisers are not willing to spend any amount of money to be associated with. To use a trivial example, imagine if the KKK had a YouTube channel. And as a more nuanced example, this woman's channel. There is some minimum cost per view, and if the ads aren't even worth that much then there won't be any revenue, rather than a small amount.

There's been a rash of negative blowback over the past couple years for being associated with certain types of content, to the point where being monetizable at all is increasingly not the default state. Advertisers would rather not risk it.