This is anecdotal but I feel it's much more common than we talk about: when I was in ad tech, I saw first hand how advertisers would spend money on impressions for the sole purpose of being able to report to their higher-ups how many impressions and click they got. Particularly for brands where conversion attribution was difficult to impossible.

The media agencies spending on be half of the brand also have virtually no reason to NOT spend ad dollars on fraudulent clicks and impressions - again, they're just trying to get paid.

And of course the publisher doesn't care if the impressions are fraudulent.

Basically, there is a very large rigged economy in ad spending that both sides perpetuate for their own good.