Fraudsters might not be able to as easily game Facebook since they don't pay per 1000 views or whatever - but they can still drive tons of traffic to websites filled with ads, scams, etc, and they can also be making money to get fake engagement to wannabe influencers, politicians, propaganda, etc

Facebook is WAAAY too big of a business to take on this liability themselves, and try to keep it secret.

They'd face securities fraud from their investors, likely huge government fines, and lawsuits from all of their advertisers.

Idly sitting by and letting the numbers pump a little higher is less of a problem.