There might be things Meta isn’t allowed to do with WhatsApp. Also the concept of account is bit blurry on WhatsApp because you basically login with your phone number and an SMS code sent to your device. This wouldn’t work as well for a service that can be used elsewhere.

In my opinion, Signal isn’t trying to be a Matrix alternative. Anonymity between users isn’t their objective, they mostly want to be a none profit alternative to WhatsApp, Messenger, etc… with strong E2E encryption. Phone numbers is ultimately the best way to discover new users as soon as you install the app.

I’m fine about ads on Signal. It is a none profit. 😉