But they do sync. They just don’t keep messaging history, which is, as you say, by design. Signal doesn’t keep copies of your messages so they cannot give you old message history if you connect your account to a new device.
That’s true, but once you trust a new device, there’s no reason the authority (your phone that has all history) couldn’t transfer the history over to the new client.
I get it would add some complexity, but it could be done in a secure and private way.
I feel like that is also by design. If your account is compromised, you wouldn’t want them to be able to pull messages from your existing devices. It kinda defeats the purpose of them not being stored on the servers.