Ad fraud has been addressed by successive rounds of technical countermeasures and continues at scale. The persistence follows from the economics rather than from any failure of the tools.
The incentive is enormous and the cost is low
Fabricated traffic converts advertising budget into revenue for whoever generates it, with no product to make and no customer to satisfy.
The infrastructure required is cheap and widely available, and the same setup can be pointed at any advertiser.
Against that, detection has to be built, maintained and paid for by parties who gain nothing when it works and lose nothing visible when it fails.
Detection is necessarily retrospective
Fraud is identified by patterns, and a pattern only becomes visible after enough instances have occurred to form one.
Any new method therefore operates freely during the period before it is characterised, which is the period in which it is most profitable.
Operators expect this and treat each method as disposable, retiring it once detection catches up and deploying the next.
The defender has to cover every route
Fraud can be introduced at the traffic source, in the ad server, in the measurement layer, or by misrepresenting the identity of a site or app.
A defender must address all of these continuously. An operator needs only one route that currently works.
That asymmetry is the fundamental reason the problem does not close, and it holds regardless of how good any individual countermeasure is.
Parts of the supply chain are not motivated to stop it
Intermediaries earn a percentage of spend, and fraudulent spend pays the same percentage as legitimate spend.
Removing supply reduces revenue immediately, while the benefit accrues to advertisers over a longer period and is difficult to demonstrate.
This is not usually deliberate complicity, it is an absence of incentive, and it explains why enforcement is stronger where advertisers audit than where they do not.
What actually reduces exposure
Buying less inventory from fewer sources is the most effective single measure, because most fraud enters through long, opaque supply paths.
Direct and deal-based buying reduces the number of intermediaries between the advertiser and the publisher, which removes most of the opportunity.
Measuring outcomes further down the funnel also helps, since fabricated traffic can produce impressions and clicks convincingly and almost never produces revenue.