Why is it worth learning from history?

"History is the best help, being a record of how things usually go wrong."

Why is it hard? Why are the lessons incomplete?

"Many documents are written to deceive or conceal. Moreover, the struggles that go on behind the scenes, and largely determine the issue, are rarely recorded in documents."

Besides being incomplete, the lessons are often deliberately misleading.

“We learn too that nothing has aided the persistence of falsehood, and the evils resulting from it, more than the unwillingness of good people to admit the truth when it was disturbing to their comfortable assurance.”

With time, we forget about incompleteness and purposeful deceit. We not so much don't take it into account, as we don't see it at all.

“The most dangerous error is failure to recognize our own tendency to error. That failure is a common affliction of authority.”

And that's why we don't learn from history.