Is it because I'm mean? Because I'm trying to 'teach you a lesson'? No! I work hard to be equitable to all my students. If I give a boon to one student, such as a makeup test, then to be perfectly fair I must offer a makeup test to ALL my students. A quick multiplication of all my students times all possible tests demonstrates that I could potentially have to write hundreds of tests each semester, something for which I will not be liable. Furthermore, if I were to give the same test as a makeup before the class there is a security risk: some students may learn of the exam questions before the rest and have an unfair advantage. Nor can I offer the test after the class takes it, because then the student for whom I'm doing a favor would have more time to prepare; again, an unfair advantage. Finally, there are a limited number of 'easy' questions, so chances are any makeup test would be significantly more difficult than the test given in class. This would be just as inequitable as giving some students favors that the other students don't receive.
The fairest way to offer exams and/or quizzes is the way I'm doing it now, the option depending on the class. This allows everyone to screw up one quiz/test or allows for one emergency during a quiz/test day per semester. Why not two drops, you ask? Because if you have two emergencies during the semester, both occurring on test days, then something more important is going on in your life that you need to address above and beyond this class. And if you screw up two quizzes/tests, well, let's just say there are academic issues that no number of makeups will alleviate.