Privacy

My thinking also. Corps/gov can also link identity to emails of course, but it's much harder due to email aliasing and the ease of new account creation.

Mobile phone numbers are far more personal - people generally only have one or two max and generally keep them for very long periods.

If its the true reason, it would make Signal not much better than Meta's WhatsApp, which gleans value from its users by noting metadata tying users to one-another by who they contact, when, and how often to extrapolate social circles and relationships. But Meta goes further in I think also tracking location, etc, and obviously has much more personal data in the linked phone numbers of many FB/Insta accounts.. Signal could potentially be doing some of that to a degree with IP geolocation... Not great.

TLDR: its the one thing stopping me from trusting Signal entirely as a benevolent actor - they want and 'need' your personal phone number. I use it still, as its the best available mainstream option, but I mention this concern when recommending it to those seeking privacy.