India's UPI is also extremely quick and easy to use - instant transfers with just the person's phone number or via a QR code or via a UPI id which looks like an email id.
We are talking about 19-20 billion transactions per month.
Apart from UPI, there are other interbank transfers methods such as NEFT, IMPS, RTGS etc. All quite convenient and easy to use.
Online UPI is IMPS, with a good UX
In some setups UPI doesn't even internet! Just an SMS is sufficient
I can see the usability aspect, but using an insecure protocol like SMS for something like payment sounds like an absolutely awful idea.
RCS might work as it's a bit more secure (and more importantly: doesn't have a 2G/3G compatible version that criminals might trivially abuse); it even has an optional money sending API in its protocol.
People who are using upi over sms don't have a lot of money. Scamming them isn't worth the trouble.
There is UPI wallet (preload your wallet) that require no internet. But this still require a smartphone.
The SMS option is mainly to avoid requiring a smartphone. There is a huge population that do not have a smartphone.