[rmah]

I've seen the high school world history books of my nieces and nephews and all of those (except maybe lithuania), were at least mentioned in US schools. Other civilizations ranging from the maya to the khmer were also included. By necessity, none of them are examined in depth but they all got at least a few pages.

The simple fact is that pre-industrial history has had literally thousands of different thriving civilizations around the world. It's simply not possible to even briefly cover anything but a fraction of the more "important" in a non-university setting.