I'm not sure how true this perception is in more recent years. Many popular sites, with enormous traffic volumes that could drive digital impression ad revenue, are instead pushing subscriptions or other monetization models.

For instance, the New York Times makes — by far — more money on digital subscriptions than digital advertising. Digital advertising revenues are also declining for them.

Another example is Spotify, where ad revenue from their ad-supported tier did not cover their operational costs and now represents around only a tenth of their revenue compared to subscriptions.

The exceptions to this are generally search and social media sites, where the product for sale on these sites are the users themselves. They're just advertising platforms, which of course make their money from digital advertising.

So I'd say one issue with digital advertising is that it often does not pay the bills for the site owner. Its value is tied to its ability to convert visitors to buyers, but it has to be ramped up to such an extreme level it instead only creates bad experiences.