Even the two examples you give which we would think would be easiest to fact check accurately, "the conclusion of a scientific study, the name of a person, and

Even the two examples you give which we would think would be easiest to fact check accurately, "the conclusion of a scientific study, the name of a person, and exact phrase that was said" are not always. With the scientific studies, we see here on HN all the time how studies, viewed from various angles, can be contentious and not easy to decide with certainty about. And surprisingly, quotes and "what I saw" situations often do get remembered differently for two people observing the same event. There's a lot of room here for some seemingly simple facts to be very difficult to know the actual fact of.