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.