Others like Threema and SimpleX don't require them. Theyre auto generated. I imagine it would be hard to have a public key repository without some sort of identifying info.
Discussion
Ya, signal is an opensource codebase that is forked off early versions of Whatsapp. The author didn't like how WhatsApp was going so he created signal. You can run your own signal server, but you'd have to also modify the signal app to talk to it. It's all still opensource. Ya the best they could do at the time was the phone # thing, now we have new paradigms rolling out with simplex and stuff.