> Why isn't this the default behavior to use this UDS?
Because it’s insecure to use on multiuser systems, as it presents an opportunistic access to remote systems for root users on your local system: root can read and write into your UDS too.
As a user, you have to explicitly opt into this scenario if you deem it acceptable.
I don't think that's the reason. root can theoretically do everything and not much is protected from root. root can su to your account and make a new SSH connection. root can replace the ssh command with one that copies their public key before opening a shell.