Right, but if millions of people suddenly began clicking on every ad they see, I think that would create a big problem. Clearly they lie about their numbers, and they probably engage with their own bots, but I feel like adding millions of clicks per minute for months would be difficult to mitigate.
Enough people use ad blockers that it's worthwhile for a lot of sites to beg you to turn them off, or maybe to just not show you the content if you're using an ad blocker.
1. most people aren't running adblockers, so them turning them off and clicking on adds would add very little noise to the data
2. the ad companies aren't exactly honest with their numbers; remember the pivot to video? that was just an out-and-out fabrication. The ad companies would just lie to their clients about click-throughs, impressions, etc.