>"draining banks in the U.S. and other countries of cash"

Cash sits in vaults. North Korea would need to physically send a team of bank robbers to extract it. The digital representation of USD is not cash, it's an entry in a ledger, and banks generally have systems that would not allow adding an entry with a sudden transfer of a large amount of money to "J. U. Kim, North Korea". That's why Bitcoin is called "digital cash".