no one is talking about the audience you are interacting with. Not everyone has an email and that does not mean they have to remain excluded.

Mobile phones are becoming a universal device, be it a rich or a poor, each one is going to have one. Hence the easiest and unique identifier is their phone number.

Now phone number is personal and could be easily tracked but the kind of audience that engages with phone number as a username is not much privy about their information when it comes to solving their problem or making their lives easier. The onus is on the system to make it entirely secure. With every data point, comes more responsibility.