Supposed according to whom?

Believe it or not, the official role of the Library of Congress is mostly to be a library serving Congress.

Their website says "The Library is the main research arm of the U.S. Congress and the home of the U.S. Copyright Office."

They do keep copies of things registered for copyrights, for copyright purposes.

I don't believe there is any legislation or funding to give them the role of finding and preserving 80 year old 78s en masse.