Or, why should we? This was brought up in a lecture and we got a number of different answers, including 'to learn from the past', 'so we don't make the same mistakes', etc etc. And these are the sort of answers that most people come up with.
And we also teach in schools things like these because of the 'importance of History' and all that which may be for similar reasons.
I think the reason why we study History though as species is much, much simpler.
I've never actually been interested in history until recently when I started looking at political philosophy and trying to find examples of different principles/systems working (Or not) in action.