To sell attention on a large scale, ad inventory had to be standardized and abstracted. “The amorphous, shapeless concept of attention has been transformed into discrete, comparable pieces that can be captured, priced, and sold,” Hwang writes. But losing the context of where that attention comes from can lead to “irrational levels of market confidence, a regular feature of financial crises going back hundreds of years,” because the worsening foundations of those markets become harder to see or easier to ignore. For example, even as the value of the digital ad industry was continuing to rise, the average clickthrough rate on Google’s display ads fell to 0.46% in 2018, ad fraud was expected to jump 21% to $42 billion in 2019, and a Google study found 56% of its display ads may not even be seen by a human. These stats suggest the product being sold is not nearly as effective or valuable as many purchasers of digital ads believe it to be.

This seems to be the main reason, basically suggesting that online advertising is in a bubble.