That isn't why Signal uses phone numbers. It uses phone numbers because users have a local list of phone numbers they communicate with and Signal can use that t

That isn't why Signal uses phone numbers. It uses phone numbers because users have a local list of phone numbers they communicate with and Signal can use that to determine if people you communicate with are on Signal. If there was a reliable local list of email addresses they could use that but most people use cloud email now and don't have their email addresses they communicate with on their device.