On the other side, as a personal user of SSH with basically one person to worry about, the effort of setting up a certificate seems like just a waste versus the existing key-based infrastructure; I don't understand at all what attack it would prevent or what convenience it would provide for the cost of learning it.
A general principle is you get good at what you do a lot, and you aren't good at what you do rarely. Key-based management has its issues, but people have been doing it for a while and they're good at it. Not necessarily "great" and not necessarily "unable to be improved", but they're good at it. People understand it and know how to use it. Nobody uses certificates, so nobody understands how they work or what to do with them, and they have the weakness I cited above. So, even though in a perfect world certificates may be superior, in the world we live in now, the competition between the existing polished infrastructure for pure key-based management is pretty good, even if you think it's a polished turd, and the infrastructure for SSH certs is pretty bad, even if you think it's an uncut diamond in the rough. It's difficult to look at that situation and honestly choose to proceed with the certificates.
As others point out, the cloud services are proceeding in other directions as well which will probably continue to make using SSH certificates directly a very difficult choice to practically justify.