Best bet, they haven't spent any significant resources on this case in decades. With no credible leads, what would any assigned staff be investigating?
Declaring the case closed is an administrative issue, nothing more.
If this case was still active, the staff would probably be involved in monitoring the banking system for the serial numbers of the bills given to the hijacker, as well as going over any seizures that included hard currency (for example: drug busts and civil forfeitures across the country; not a small task).
Theoretically, the job could be done by a single python script with a direct line to some central currency database fed by all banks and local law enforcement but with government, everything is more complicated.
Declaring the case closed is an administrative issue, nothing more.