It's a bit disappointing, but this is fortunately one of the areas where mods can deliver incredible value to a game.
I would love to see cultural variants of all units throughout the whole span of the game (come on, Mayan/Aztec giant death robots!), but I can also understand that this is a fairly minor thing that carries with it a pretty large expense for the developer.
Indeed! In fact one of the reasons I joined this forum in the first place was to hunt for cultural diversity mods for Civ 4!
Vanilla Civ 6 uses, I think, culturally-specific details such as helmets, hairstyles, weapons, shields, etc to show diversity in its ancient units. This seems a lot less labour intensive than bespoke unit models. Unfortunately few of the units added in the subsequent expansions really took advantage of this. Coursers and Skirmishers at least use these details, and siege units repurpose unit graphics from melee units.
But Pike and Shot and Cuirassier units, as well as these new melee units just have the one model with no cultural details.