They're not the only banks in the world, just among the safest. We can co-opt or browbeat EU banks, Japan, Korea, and little third-world tax shelter banks like those in the Bahamas, but 'Following the money trail' into the Continental Far East, the Islamic world, China, Russia, SE Asia, is damn' near impossible to do accurately and thoroughly and then impossible to impose functional sanctions on the countries where they sit...well, unless you want to crash the world economy, anyway.