Perhaps it's more accurate to write that incest offences are common amongst reported AMSC?
Or maybe Finnish culture is different to US culture in a way that shifts the numbers.
Or maybe incest survivors didn't answer honestly in the survey.
Anecdotally on reddit, a LOT of people talking about being sexually abused as children talk about a familial perpetrator.
Actually the number of incidents, in general, seems to be lower than what "common knowledge" says. Roughly 3% of boys and 12% of girls- much lower than the "1 in 4 girls and 1 in 6 boys" that gets thrown around.
https://www.d2l.org/child-sexual-abuse/prevalence/
About 1 in 7 girls and 1 in 25 boys will be sexually abused before they turn 18*
This seems to match the Finnish data a little better.
Regardless, I guess we're never going to have clear "answers" to think. Any kind of reform will depend to a degree on ideology and faith.