1) I see a sudden spike in my daily spend and think "Yay, I'm going to get more customers!"
2) I go to Analytics and see a wave of people who spent seconds on the website and did not convert to the trial.
3) Google bills my credit card for hundreds of dollars.
This has actually happened, although the specific incentive to click fraud was different. I was sixteen flavors of pissed.
Google keeping me unpissed is worth $10,000 a year. I'm a wee little customer. The whackamole sites are worth a few tens of dollars a year when behaving normally. What do you expect Google to do?