Why do bank transfers still take days to complete?

There are many reasons. Interest (or float, interest on free money every day) is what banks love. However, of late, in a competitive world and where the consumers are becoming more educated, not to mention, the power of social media, consumer reporting bureaus, and regulations, it is becoming harder and harder for banks to justify such holds (as they are now audited).

The reason we see more often now than anything else is, compliance. This is especially true when doing cross-border money transfers. Either the correspondent bank one of the parties involved in the transfer, would like to obtain more information about the nature of the transfer and this can take time. If a correspondent bank asks for questions /clarifications on the transfer (purpose, full name, occupation, city of birth, etc.) to better identify the sender (or receiver), then this takes a few days for the information to be requested and funneled back to the correspondent bank.