I'm guessing the fact that frost caves only generate in unloaded areas is either a technical limitation, or a choice so the caves don't ruin existing builds and terrain, and so they don't bug out our worlds. It could be a combination of both of these.

Either way, it wasn't very hard for me personally to find them (first newly explored mountain biome, and two frost caves). Just make sure you find large biomes with obsidian cause that's where they can spawn.

Also I'd like to mention that we are lucky this can occur at all, from most of the games with frequent updates and old worlds like Terraria or Minecraft, new things don't spawn in old worlds.