As the middle man Google benefits from ad-fraud and has few incentives to really stop it. Ad-blocking is a real problem for them however, and they have huge incentives to prevent that. Ignore what they say - that's what WEI is actually about.
Googles main business is ads on google search. Here they aren't a middle man.
Companies give google $X, and hopefully sell Y extra products. X/Y is the cost per sale. Google competes with other advert forms (eg. TV/radio/newspaper ads) on that X/Y number.
If there is ad fraud, that Y number gets decreased (budget is used up on fraud that doesn't translate to sales), and their revenue decreases as advertisers spend their ad budget on other mediums.
