I think mainly because they don't have the same means to seek help or protection the way adults do.

Kids can't drive, they don't have money, they don't have the education to know their rights or research laws. The world is built for adults.

Asking parents to be that advocate for the child can be a conflict of interest if the parent is the one causing the issue. It seems like when the police investigate their own conduct.

We make different rules to protect people due to physical or mental disparity, and children and typically less physically and mentally able than most adults. And they have no financial means on top of that.

Does everything need to change? No. But adults don't need to worry about forced marriage, genital mutilation, or being beat up for me misbehaving or if someone is just in a bad mood near as much as kids need to, and again, their current first line and sometimes only line of protection may be the one doing that to them.