Because kids interact with each other at school. The whole point of rules is to affect social interaction, to shape how people’s behavior is able to affect others.

A parent choosing to use public schools doesn't get 100% control over what their child is exposed to. That "kids interact with others at school" is the point where the parents teaching in their child needs to hold up when the parent isn't there. I would think that is a large part of raising children. A parent knows as some point in the future their child will be an adult, and out of the control of the parents. Interaction with other kids at public schools is where that first is encountered.

If a parent demands 100% control of the children 24/7/365, then the choice is home schooling, and hopefully the child can afford good therapists when they are an adult to undo that damage.