Elementary level history is simplified for the sake of getting the point across to kids. While Columbus wasn't the first to hit the New World, his 'discovery' sparked the Era of Exploration and led directly into European colonialism in the western hemisphere which dramatically changed the course of history. All Leif did was kill some seals and go home. For what it's worth, I knew about Leif in 3rd or 4th grade.
Likewise, while Lincoln did not immediately address the issue of slavery in the 'neutral' Southern states, the Emancipation Proclamation led directly into the 13th Amendment.
I won't dare argue that there's inaccuracies in textbooks or that many are woefully outdated. But sometimes you brush over stuff when discussing history to 5th graders for the sake of getting a concept completed without nitpicking details.