Arguably, many people groups indigenous to what is now the US (and often times into Canada and Mexico as well) were each their own countries and sometimes joined into confederacies (for example the Iroquois Confederacy and some others). I do think indigenous voices frequently get lost (and that does need fixing), but I don't know if there's value in representing them as a single unit as though they were a single nation before. Many groups came over at different times, migrated around, etc. They're not even all in the same macro language families (and may have come from separate peopling events, but that's a whole other can of worms).