1. WhatsApp was the first widely-used such app and it was phone number-based, so all the clones use the same system.
2. Non-technical people have a hard time (read: bordering on impossible) remembering their usernames, let alone passwords, and by using the phone number as both identification and authorization, the problem is sidestepped.
3. People have an existing address book of contacts (in the form of phone numbers) in their mobile phones, that can be used to pre-populate the app's buddy list.
1. WhatsApp was the first widely-used such app and it was phone number-based, so all the clones use the same system.
2. Non-technical people have a hard time (read: bordering on impossible) remembering their usernames, let alone passwords, and by using the phone number as both identification and authorization, the problem is sidestepped.
3. People have an existing address book of contacts (in the form of phone numbers) in their mobile phones, that can be used to pre-populate the app's buddy list.