In my opinion, Signal isn’t trying to be a Matrix alternative. Anonymity between users isn’t their objective, they mostly want to be a none profit alternative to WhatsApp, Messenger, etc… with strong E2E encryption. Phone numbers is ultimately the best way to discover new users as soon as you install the app.
Because being tight is considered a beauty standard for women, not so much for men. For men this is mostly upper body muscle (shoulder, arms). A vest top would he an equivalent, and you don’t see skinny guys wearing that either.