It's a bit confusing that you can't "uninstall" Georeferencer which I think, when I was first looking at QGIS3, I took to mean that it was installed and operational. I think there is a semantic/logic issue here. Does install mean that you've downloaded the plugin or that it's installed, that is, ready for use? The word missing in the plugin manager is activate. And the word "Search" appears right above the checkboxes for activation. Also when you're looking at "Not installed" plugin list, there is a blank spot to the left of the listed items where a checkbox might or might not be. For a seemingly simple window, a lot going uninstall (although some can't be), reinstall, upgrade; then lists of plugins and a settings icon mixed into the list. A very ambitious app and these minor UI inconsistencies are somewhat unavoidable. Thanks for the great app and all the hard work.
Searching for the item in the Plugin Manager sometimes changes the behavior of the check box.