Isolation is the key...think about it, if you celebrate birthdays you would have a party, you would go to others kids parties. This could easily be upwards of 30 parties a year when kids are young. That's means hours & hours of worldly fun & association. Same with sports.
Once kids know that you don't celebrate birthdays, or Xmas, or Easter, and you can't join the netball team, or go to their house after school, and no it's not ok for their mum to pick you up etc etc friends dry up pretty fast........you have years & years of aloneness with only the KH for company.
its a miserable thing to do to a child.