In a word: Convenience. WhatsApp has a total overhead of about one minute per year. Registration requires you to enter your phone number. Once. Afterwards you d

In a word: Convenience. WhatsApp has a total overhead of about one minute per year. Registration requires you to enter your phone number. Once. Afterwards you don't have to worry about anything ever again. Maintaining your contact list is a sunk cost, since it runs off phone numbers. It just works.