It's mainly to do with time constraints. Many schools operate on an 18 week schedule. That means you have a 18 weeks to get from the Roanoke colony to the end of the Civil War for American history 1. Some schools operate on a yearly schedule meaning they have 32 weeks to teach American history but that usually just means combining American history one and American history 2 into 1 large class.

So there simply isn't time to focus on every thing in our early colonial history. Especially when it's more important for the student to learn about how the government was constructed. And many of the pre-colonial events will be covered in Western civilization or world history

Right now making my lesson plans for my upcoming classes next week I'm having to teach a chapter each week to be able to get to the Civil War by the end of the semester. I don't really have time to go in any deeper not to mention that many of the kids aren't interested.

Another thing is that High School down is meant to be a survey class. Where general information is covered. While College is meant to be where you take more in-depth study.

And that's American history now imagine trying to put over 4000 years of human civilization into 18 weeks there is no way you will be able to cover a tenth of everything.