Is it because it is ingrained/drilled into us as children?
I can remember my childhood librarian “bless her sole” miss Hob-son as if it was yesterday saying “Shh” this is a library and those signs above the door that most kids would through spit balls. ” THIS IS A QUIET ZONE”.
The fact is when we find our self in an environment that is quiet, where we can actually hear each other clearly, we tend to speak more softly.
So to help kids be more quiet we simply need to create more “QUIET ZONES”
It would be great to hear other suggestions you may have on How to keep kids quiet.