I am from the same place and what what_ever is trying to say, I think, is that in these places mobile number is pretty much the identity and username and emails are largely irrelevant for the majority and when I say that majority that's like overwhelming majority and they are coming online for the first and they really don't have time or any interest to learn about email and messaging protocols.

For example, we recently launched UPI payments - that's like a banking address sort of thing (you can send from it and request too) and you usually get your UPI address like this "9876543210@upi". The number part is simply the 10 digit mobile number and that's it.