I hate that this type of statement always pops up as soon as I see any mention of LE. While it might be true for some parts of the US (and probably other countries), it isn't true for the majority of people involved in this field. It's just incredibly hard (thankfully) to convict someone of a crime. It requires a lot of evidence and paperwork. Meanwhile there's a pretty limited amount of resources to work on this kind of thing.
It would be impossible for 'authorities' to investigate 20 million images they receive from Facebook. But implying that authorities don't investigate anything, and don't even want to, is just flat out wrong. For example, here's a story about an awful child predator that was caught in The Netherlands, because American authorities started an investigation into reported images: https://en.wikipedia.org/wiki/Amsterdam_sex_crimes_case
This case is also an example of what authorities tend to focus on, which is producers. A lot of effort is spent trying to prevent producers/abusers from causing more harm. All of this effort can't be invested in investigating brokers/spreaders of 'known' CSAM. Time is limited for everyone and it is too for authorities.
And yes, a probable majority of fraud cases don't get picked up, even if they seem obvious and easy to solve. This is because the man hours for actually getting someone convicted is huge and authorities usually focus on cases involving violence. I personally had to cut short fraud investigations, because preventing a wife from getting murdered by her husband or stopping an illegal arms shipment took preference. And I can't say I feel guilty about that.