Why can international bank transfers take days or weeks when emails move in seconds?

For banks it is a matter of having a trust relationship with another bank. If two banks that are in different countries don't have a working relationship, then they both will search for a bank that would be able to offer them a correspondent banking relationship. This is what causes the delays, the hops in between. Digital money needs to move properly from one bank to another until it arrives at its final destination. At present, depending on the countries and the correspondent banks involved, this can be done in a day or as long as 3-4 days.