Bots. If it makes you feel better, you can disable other people finding you via phone number and just give them your username. All messages are private.

Everything is a balancing act. Privacy, anonymity, and security aren't the same things. They're sometimes, and in some aspects always, difficult to achieve without compromising one of the other two.

When you add in the goal of quick, easy setup to make the service useful in the first place. Doesn't matter how good the service is at the trinity if nobody is willing to use it. Signal just errs on security first, privacy second, anonymity third.

One of the design goals is that they don't have a user database, so governments etc can't knock down their door demanding anything. By using phone numbers your "contacts" are not on their servers but local on your phone.

Signal is not perfect but we control its app, libre software. See SimpleX Chat.

Escaping WhatsApp and Discord, anti-libre software, is more important.

It's focused on ensuring there is no middleman between you and the other party, but it does not have a goal to provide anonymous messaging. Sadly.

If you want to be mainstream a) you can't have spammers, scammers, and all the other scum of the earth and b) finding your contacts in the app HAVE TO be plug and play. Literally no normie will bother adding with usernames or whatever.