> They need access to the private key for that, which ACME (and other certificate signing protocols in general) doesn't share with the CA.

Modern TLS doesn't even rely on the privacy of the private key 'as much' as it used: nowadays with (perfect) forward secrecy it's mainly used to establish trust, and after which the two parties generate transient session keys.

* https://en.wikipedia.org/wiki/Forward_secrecy

So even if the private key is compromised sometime in the future, past conversation cannot be decrypted.