Most HTTPS connections today negotiate ephemeral keys at the start of the connection, so even if an attacker has the server's private key (which the CA never sees and couldn't sell!), the attacker can't do passive listening attacks on connections using it. The attacker would have to do an active man-in-the-middle attack that rewrites the connection and swaps out the ephemeral keys with keys known to the attacker, which risks detection.

If an attacker has the CA's private key, then the attacker can mint new HTTPS certificates. They wouldn't be able to do passive listening attacks on connections, but they could use an active man-in-the-middle attack to swap out the server's certificate in the connection. However, this attack could be detected through Certificate Transparency, and the CA's leaked keys would become untrusted by browsers.