> Nobody verifies host keys,
The known_hosts file is verification of host keys. It's not verification of a host cert, which is a different thing. Most sshd instances are running on ad hoc hardware without the ability to associate them with someone a cert authority would be willing to authenticate.
Basically people running services that need cert-based authentication are already using TLS (or if they're using sshd they've locked it down appropriately). SSH is for your workstation and your RPi and whatnot.
SSH certs aren't TLS certs. Totally different format. All SSH CAs are private, you run your own CA to issue certs to devices you want to allow to connect to your server.