The economics isn't there anymore
Go up to Grass Valley and visit what used to be the Empire Mine (it's now a state park and nice for a picnic). It once was the world's deepest gold mine. It never ran out of gold. During World War II the mine was carefully mothballed and allowed to flood. Unfortunately it never became economical to de-water the mine and bring it back into production. The same holds true for the Comstock Lode under Virginia City (the silver mined there paid for the Civil War). The Comstock Lode never ran out of silver but became too expensive to mine. This same story has been repeated all across the United States. The economical mineral resources have been mined out.
Gary