You're using a messaging app that was built with the express intent of being private and encrypted.

Yes.

You're asking why you can't have a right to privacy when you use your real name as your display handle in order to hide your phone number.

I didn't ask anything. I stated it definitively.

If you then use personal details as your screen name, you can't get mad at the app for not hiding your personal details.

I've already explained this. I am not mad. I am telling you why it's a bad product for activism.

Chatting with your friends and clients isn't what this app is for.

That's...exactly what it's for. And I don't know where you got the idea that it's not. It's absurd. Certainly Snowden never said anything of the sort. Signal themselves never said anything of the sort.

There are other apps for that.

Of course there are. They're varying degrees of not private, secure, or easy to use.