Because you can’t argue that. Any other ground reason for policy can be challenged or counterargued or relies on values which are arguable.

No one is going to plainly argue “ok but how about we do not protect children?”. And if someone tries a different angle such as “this law is not really going to protect anyone and will bring a lot of problems for children and adults alike” it will be easily dismissed as “you insidious snake, why do you want to hurt children?! Don’t sabotage child protection!”. Which autokills conversation.