I think to your question about why banks don't do it themselves is because it's not their business. Banks are in business to make money, not create new technology. Maybe a handful of the largest banks could afford a dedicated department to creating their own blockchain but why have the expense if someone has already done it and it's cheaper than current alternatives like SWIFT transfers. It takes away some of the risk for a bank to have someone else responsible for making the transfer system work and maintaining the system.