I don't think it as rare as people like to believe. Or certainly was not in the past. Current awareness and attitudes may now ensure it is more difficult for certain institutions to be used as opportunities.
But along with this there is the scandal over here of the Benedictines forfeiting the right to run a school because of systematic and widespread abuse. When I was 'kicked out' of my school for propositioning a monk (I was 18) their brutal reaction, it turned out, was because they were entirely aware of at least two monks abusing boys at the time and were intent on keeping it covered up and terrified my behaviour would draw attention to it . This entailed allowing one of them to remain as head of the junior school and thus have access to 8 year olds who he abused regularly. I was just fed into their hypocritical smokescreen. Both monks have now done time but it took 20 years for that to happen.
With Penn State it seems they were willing to cover it up for fear of risking continued sporting success.
Appalling.
But it seems those in certain institutions will always be willing to cover up abuse if they think they can.

Of people I know I am aware of 4 who have been abused. Two are women who were abused by their fathers, another by her step father.
The family being the one institution not easily monitored.