Fraud at such a massive scale (+$16 billion ) is made possible by the lack of transparency in the digital advertising supply chain. Advertisers often don't know which websites the ads they buy end up on, leaving room for malicious publishers with mostly bot traffic to profit at their expense.
This problem is exacerbated by the large number of intermediaries in a typical programmatic advertising supply chain. A single ad impression can transact between dozens of parties that sit between the advertiser and the publisher. These intermediaries, who are often paid on a per impression basis, may have limited incentive to report questionable traffic, since anybody who reports fraud down the line forfeits their own revenue by doing so.