1. That their messages won't be lost when they migrate between devices.
2. That their messages won't be lost when their device is stolen and they set up the new one from nothing but a password.
3. That Apple's password recovery flows work like any other password recovery flows, AKA that forgetting your password is a minor inconvenience, to be overcome at the Apple Store at worst, not a data loss disaster.
4. That they don't have to spend $$$ on some strange device called a "Yoobby Key", which they don't understand and will lose anyway.
There's no way to satisfy those demands and have your desired level of security, hence why iCloud backup encryption is a strictly opt-in feature.
There are tradeoffs to be made here, and Signal made different tradeoffs, which makes it significantly more secure but also significantly more annoying to use for somebody whose main life interest isn't figuring out why tech works the way it does. Apple does the best it can under the constraints they are given.