The copyright office does not try to collect and preserve, en masse, old recordings from before they started taking deposits of audio recordings for copyright registration. (1972 prob? sound recordings didn't have federal copyright protection in the USA before 1972) There is no my knowledge no legislation authorizing that or funding from the government for it.
The Lib of Congress (probably not in the copyright office part of it) does have some historical audio recordings in their collection, including digitizations, but I don't think on this scale.
Why do you keep stating things you can’t find untrue with a moments search?
Yes, the Library of Congress actively collects old audio (and even has 450,000 of those old 78s you claimed they don’t gather, and around another million unpublished ones). They have funding and mandates to do so.
This entire thread is you posting opinion as fact, but these opinions are demonstrably false. Before posting things as true please do some due diligence.
So “supposed according to whom” is according to law and all of history.