I think this misconstrus what public domain is.

It provides a freedom to circulate, but not access to the material. It is not a public owned resource.

Turning a copy over to the public or state might be an interesting requirement for obtaining a copyright, but instituting that fix for new works now would have a 70 year lag time.

Think of it this way, if I copyright a book and put it in my dresser for 70 years, that doesn't give the public the right to access it or come into my house and scan it after expiry

>Turning a copy over to the public or state might be an interesting requirement for obtaining a copyright

This has been a requirement in the US since 1790. It is called mandatory deposit: https://www.copyright.gov/help/faq/mandatory_deposit.html

They don't keep every book though.