I’m pretty sure it’s for security reasons, it allows the app to ensure the code comes from a correct Signal phone number and not from a third party actor. It’s hand holding yes but it’s a trade off in favor of non-technical users.
Signal always has excuses, "we can't add usernames because we don't know how to make them secure"
But if they sent you the code, even if a third party actor could somehow see it and hand it to you instead -- so what? What's the difference?