Great question. This is pretty unfortunate, data mining secure communications removes much of the value. Signal sold out a long time ago, not sure of another 'verified secure' platform.
My understanding is that their intended audience is the general public, not crypto-security geeks, and as part of that they wanted integration with existing address books. With a small team, developing all the security and usability was more important than eliminating the phone number piece.
They apparently don't retain any data but the phone number, and I think the registration date and last logon date.