During an open book test, students tend to believe that studying or memorizing is unnecessary because they "can just copy straight from the book. This is a myth for several reasons. For example, exams might have a word limit, which means that you cannot copy all the information that the book presents. Instead, you have to select the most important points, which could take you a long time if you are not familiar with the topic. Also sometimes they may word the question differently, asking to analyze or asking you to solve instead of define.
Going on the assumption that it's about costs greater than benefits or something similar: People will do less of an activity when the costs outweigh the benefits (or the cons outweigh the pros) because it simply isn't worth doing; it causes them more harm than good
