We're not accusing Shakespeare of false advertising or anything, but the full title of this play is just a tad misleading. Sure, it's definitely a play that focuses on King John I of England, and, sure, the guy definitely dies at the end of it. But when you stick the word "Life" in there, we're sort of expecting to learn a bit about his childhood, how he came of age, his love-life, and so on. Right?
Well, this isn't a J.D. Salinger novel, so that's not exactly what we get. Basically, the play just follows King John's career from a certain point in his adulthood—when he goes to war with the King of France and his allies—until he suddenly croaks after being poisoned by a monk.
Maybe the point is that Shakespeare is trying to give us a symbolic episode from King John's life—you know, something that can sum up who this guy was and what he did throughout his life. We don't even need to see Robin Hood or the signing of the Magna Carta, because we can already see from the events in this play just what kind of a guy John was and what his problems were.