Caches definitely are real. People did hide stuff, there was a time when people stopped trusting banks, or, had little access to banking services. Sometimes, they cached money for security purposes. If their farmstead was burglarized, they might get some worldly possessions, but they wouldn't get the money saved for the mortgage note, or next seasons seed bill.
Obviously, almost everyone went back to retrieve their caches, which means that whatever remains 'out there' is going to be a result of a pretty unusual chain of circumstances- which in turn means that sizable caches are going to be very, very, very rare.
As others mentioned, you also have to consider what sort of people were liquid enough to cache meaningful sums of money. There seems to be this bizarre, romanticized image of hardscrabble 'Depressioners' working hard for a meager sum, only to go run off and bury it somewhere. Possible, but pretty damn unlikely.
I'm guessing the best-case scenario for a gold coin cache is an eccentric, wealthy merchant who died suddenly in the 30's... or, remote places used by bootleggers.