This is because history really doesn't just deal with the past. History is not about what happened in the past. It is, rather, about why things happened in the past. And that is something that is really important for us in the present.
For example, one issue in history is "why was the Great Depression so bad?" Economic historians think that they know why that was. Therefore, when the financial crisis hit in 2008, they felt that they knew what to do about it to prevent another Depression. Of course, we can't know for sure if they were right, but at least we didn't get another Great Depression.
History, then, is important to us today because it can teach us lessons. If we can learn from the mistakes made in the past, we don't have to (as the saying goes) repeat them in the present.