Why do you think that is? Do you realize how much pressure a textbook writer or school district would face for teaching, in so many words, that the Pilgrims tried setting up a theocracy, and the problems inherent in that led the Founding Fathers to build a secular government? How dare the schools suggest such a thing! Next they'll try and teach kids that evolution is true.
The reason is the same as in math, physics, chemistry, English. It's more important to cover a larger amount of material than it is to actually have students learn useful knowledge in a subject. There are lots of high school students who could tell you the dates of Napoleon's reign, the dates of Otto von Bismark's reign, the dates of WWI, WWII, and the cold war, but only a tiny fraction of those students could explain with clarity and insight how those disparate events are related and linked. Yet it's that knowledge, more than X significant event happened during year Y at location Z, which makes knowledge of history valuable.
