@enigmatico all IMs at the time expected you to authorize people who wanted to add you, beginning with ICQ which was the first instant messenger.

The change to phone numbers for identity and removal of your ability to decline being added was part of the 'frictionless' fad of making UI as simple and brainless as possible even if it meant losing functionality.

For WhatsApp the historical reason for doing this was because it was pushed as an SMS replacement so it made sense to use phone numbers and not ask for permission because it was just using your contact list.

Unfortunately the ease of setting it up made its popularity explode so hard that everyone copied the identity and UI approach for everything else and here we are.