Yes, because they have the real thing: Instant and free cross-country transfers by bank account number. Why use something strictly less powerful?

A phone number based transfer service can be implemented as a directory service on top of that much more easily.

You mean like MPesa, the exploitative pseudo banking system popular in African countries where mobile phones are more likely to be available than electricity and running water?

What would be the benefit of that if you don’t mind me asking?