The problem is that you can issue two certificates for one domain from two different CAs. Which one is valid?
If you only have one of the certificates, you also can’t know that another exists to warn the user that they might be connecting to a government-operated middleman.
The problem with a government issued CA being trusted is that the government can now issue whatever certificates they want for any website, and then all they need to do is force your traffic to pass through their servers first.
And no they don’t even need to make fake website clones, they have you connect to their proxy server which has a valid cert, then they have everything plaintext to save off to look at, and they forward the connection to the original website. Reverse proxy servers to accomplish this take minutes to set up.