Funny/stupid anecdote: a bunch of my kids' friends have the same phone unlock PIN as me because I set my son's new phone PIN the same as mine so he would also be able to unlock my phone if necessary.
When his friends started getting phones as well, they copied his. This has migrated through some of the friends' siblings as well.
One of my kids friends uses the same pin as I had when he was a kid. I would give him and my kid my debit card and they would go get pizza if they cleaned my office.
When his friends started getting phones as well, they copied his. This has migrated through some of the friends' siblings as well.