Well, there's certainly no objectivity in academic sociology or history, but there's a culture of trying to build narratives and justify viewpoints through richer collections of evidence: multiple testimonies, contemporaneous records and primary ephemera, prior publications, etc

In contrast, memoir is often simply held to the standard of "is there strong and ready evidence against this claim, or can it at least hold up as the author's subjective interpretation of some event" because it's purpose and history as a genre is entirely different. Its audiences aren't academic peers hungry for material against which they can safely anchor their own work, but tens of thousands of airplane readers who are hungry for titillation or personal relatability.