Why do historians need to cross-check and compare primary source documents?

Because there are certain things that were thought years ago, but they are now cleared up. (i.e. long ago, scientists thought that aliens built the pyramids because of its complexity but now we know that it is just pseudoscience and not real  because there is nothing to back it up)

People cheat because they’re desperate, they procrastinate, they don’t know how to do things.