Well as a bank, you sign a bunch of agreements and promise to comply with certain regulations. If a bank decided to use cryptocurrencies to transfer money overseas to decrease SWIFT fees, that bank would be cut off from the international banking system and dollar system overnight. So it's not as simple as you put it.
Surely if it was a bank like HSBC, with branches everywhere on earth, moving money via cryptocurrency would be easy as it would happen from say HSBC China to HSBC UK. Legally they are separately incorporated banks regulated by the local regulator in each country, but there is nothing stopping them moving money from one to another with cryptocurrency.