Foolish and sensible consistencies in grading
The practical problem with reading more than 10 or 15 student exams or papers is that one has to balance the need for consistency with the need for sanity. Even with a good bit of experience and a grading scheme (or rubric, or what-have-you—a set of guidelines for grading), there is always the good chance that one’s standards will change from the beginning of a batch to the end. As a colleague tells me, sometimes the paper that you cracked down on for minor factual problems looks a lot better by the bottom of the pile. Conscientious teachers are aware of that and will reread some papers to make sure that the grading standards haven’t changed.
But the deeper grading problem is a simpler physical one: reading student work carefully is exhausting.