The sheer volume of paper out there means that there's simply no way that archivists have been able to go through everything. Some boxes haven't been opened since the 1800s, and we may never have any idea what these things are. See, archivists need permission to go through material like that. To do so, you need to tell the higher-ups specifically where you want to look and what you're looking for. You can't simply start randomly spelunking in piles of government papers -- the files will get messed up even worse than they are now. Somewhere in our records are papers that could change what we know about the history of our country. Every archivist knows this. But we need to get through everything first, and with mundane governmental papers taking priority (looking at you, Veterans Affairs), archivists rarely get the chance to discover new things.
"America isn't ready to learn about West Dakota."
But archivists do make discoveries all the time. They've found everything from a letter from the FBI to MLK to unopened letters from Axis families wondering how their sons were doing in POW camps. Sometimes letters are immediately classified, and other times they are brought straight to the public's attention. But for the most part, we don't find letters and documents like these on purpose -- it's usually totally by accident.