Earlier quoted context omitted.
While I doubt malicious intent on FB's part there's certainly a financial incentive to ignore this problem. How would their earnings reports change if they stopped charging at least some advertisers for 80% of clicks?
It would be a very, very short term financial incentive. Advertisers monitor to see performance on the other end, and when the campaigns don't yield actual success they move on. The only real exception to this are mega-brands that advertise just to build namespace. Everyone else expects direct action from some hefty percentage of their ads.
Imagine a 100 person company with 5 people dedicated to advertising. Leadership says to increase on-line presence. Some recent college grad on the advertising team buys FB ads. Next month leadership is pleased that they're advertising on Facebook. Also leadership is pleased that sales increased x%. The FB ads could be completely worthless. Some non-trivial percentage of clicks could be from bots and no one would notice.