You can use a username only for finding and adding friends, you only need the phone number to create an account. That's probably because Signal started as an alternative to Messages (or whatever it was called back then), so you could send SMS if you wanted, or secure messages to friends w/ Signal. The whole point was to be a gentle transition from SMS to private messaging. However, they eventually dropped the SMS feature, but it seems they kept the phone number as username thing.

It kind of sucks, but I think that's a reasonable limitation since the vast majority of people using this service will have a phone number. You could probably even sign up for a free trial of something (e.g. Google Fi) to sign up for Signal, set up the username, and then drop the phone number service. I don't know if there are any problems with this, but I don't think they do anything with your phone number after everything is set up.