I don't think it's an English thing but I think there are definitely cultural aspects for some people that prevent it.
Eg, I have a 30something Muslim friend who experiences familial abuse and have a 50something Irish Catholic friend who also experiences ongoing familial abuse. In both cases, the abuse is significant - emotional, financial and physical - and started when they were young. It's all either of them have ever known.
One has had years of therapy to deal with her feelings around it and it's had a significant impact on her life. The other is too scared/loyal to go for therapy and be honest about how he feels. Both of them would love to he free of it but both have also said that they will never reduce contact with their families for religious/cultural reasons.
As for those who say "only on MN". A lot of the time, you just wouldn't know. Every single one of my colleagues could probably say they don't know anyone who is NC with family. But they do because I am.
Most of my friends would say the same. I've only told a small handful of people in the last 13 years.
My brother tells people who ask about family that both of our parents are dead (one is).
In many cases, you just simply wouldn't know.