Google's AdSpam/fraud/bot-prevention team was, when I worked there, world class and fairly well funded, took their job seriously, and had access to all the data. It's an existential threat to the ad business, because if Google gets a reputation for being full of bots/spam, then the advertisers will bid lower per click/conversion to compensate, which means that legitimate website publishers will get paid less and go to other networks, which is a feedback loop that leads to the entire market collapsing (see also: https://en.wikipedia.org/wiki/The_Market_for_Lemons). It's absolutely worth refunding/zero-rating huge amounts of advertiser spend to avoid that situation, and they do.
It's not that they're not trying, it's just a very hard problem.
It's not that they're not trying, it's just a very hard problem.