Amethyst Geodes won't generate in superflat worlds, plain and simple. Of course, it would be weird if they generated in void worlds, by why wouldn't the option be there? More importantly, they don't generate when using the 'Tunnelers' Dream' preset, while other ores like coal do generate.

There should be an option to forcefully turn on or off the generation of Amethyst Geodes, for any world type. There should probably be an option in the experimental world generation settings.

I've attached a screenshot that shows me using the Tunnelers' Dream superflat preset, and how underground water & lava lakes generate but there are no Amethyst Geodes.