Only because they've developed techniques for reasoning about documents. That'll happen for other material too. No historian reads a letter from 1923 and forget

Only because they've developed techniques for reasoning about documents. That'll happen for other material too. No historian reads a letter from 1923 and forgets that it was written by a person who has opinions, biases, and narratives. My wife is a historian. A nontrivial amount of her work is reading about petty business squabbles from centuries past in various letters to state actors. Those letters contain narratives, biases, goals, and even just straight up lies. And dealing with this is a basic part of being a historian. A historian who reads a postcard from a WW1 soldier and determines that it is a transparant depiction of everyday life without further analysis is the sort of person I'd expect to never get a faculty position.