Most major banks are still using systems that were built 40 years ago for this task. Swift, MoneyGram, and Western Union are just some examples of slow, expensive, and relatively limited systems that financial services use to transfer money.
On top of that, not all of the banks are connected via the same network. So, in many cases, you don’t even have a direct line between two banks when they need to transfer money from one account to the other.
For bank A to send money to bank B that doesn’t have any direct relationship, more often than not, it will have to go through several intermediary banks, searching for common network connections between each other to clear a path for the money.
That’s why international wire transfers are so slow and costly. Each bank along the way takes time to process the transaction and a fee for servicing the process.
In some cases, bank transfers must involve currency conversions, which make things even more problematic and expensive.
The financial institutions make trillions of dollars collectively in transaction fees. This has created even lower incentive for them to innovate.