Another aspect is the social graph. It’s targeted for normies to easily switch to.
Very few people want to install a communication app, open the compose screen for the first time, and be met by an empty list of who they can communicate with.
https://signal.org/blog/private-contact-discovery/
By using phone numbers, you can message your friends without needing to have them all register usernames and tell them to you. It also means Signal doesn’t need to keep a copy of your contact list on their servers, everyone has their local contact list.
This means private messages for loads of people, their goal.
Hey, we know this account sent this message and you have to give us everything you have about this account
It’s a bit backwards, since your account is your phone number, the agency would be asking “give us everything you have from this number”. They’ve already IDed you at that point.
I’ve always just wiped my work laptop and installed Linux.