What is the reason that once a student has passed a university course, the student cannot take the course again?
In the United States, institutions that have such a rule do so primarily because of the requirement of satisfactory academic progress, or SAP, that is imposed by some, perhaps many, forms of financial aid. "Progress" means accruing the credit hours necessary to earn one's degree, and students who repeat courses for which they've already earned a satisfactory grade are not progressing (as fast) toward graduation. Students who do that more than a couple of times may have a high GPA, but unsatisfactory academic progress, and may lose their financial aid.
Edited to add: Sadly, and to me astonishingly and dismayingly, the subtleties of how financial aid works seem to escape many students. They overlook the SAP requirement entirely or decide they will somehow "catch up later." In the United States, it is difficult or impossible to have different rules for similar groups of people, so we generally can't have one rule for students with financial aid and another for those without. The result is that, even if it makes no difference to your financial situation, in many institutions you can't repeat a class in which you've earned a satisfactory grade.
Summary: this rule is about efficient allocation of resources.
In an ideal world, people would study whatever subjects they felt were useful or enjoyable for them to study, however many times and for as long as it took them to satisfy their curiosity and reach a level of mastery of the subject that they are happy with.
Sadly, we do not live in an ideal world. Two specific ways in which our world is not ideal are:
In our world, education is a scarce, and very expensive, resource. At any university, there is only a finite number of classes that can be offered to a finite number of students, and the demand quite frequently exceeds the supply. This creates a pressure to strive for efficiency in various ways, and in particular to impose mechanisms that avoid a (relative) waste by teaching people things that they already know.
In our world, education, and grades in particular, have very significant consequences for people’s future, greatly affecting one’s chances of getting a scholarship that enables them to stay in school, getting a good job, making money, and ultimately, having a good life.
In some countries (the U.S. among them, in my opinion), this creates a very intense level of competition over what is already a scarce resource, and results in highly distorted incentives that cause students’ decisions regarding topics to study and how many times to study them to be motivated by highly non-ideal considerations having nothing to do with intellectual curiosity: e.g., it is very common to see students taking specific courses, or taking courses with a specific professor, or wanting to retake a course they did poorly in the first time, for a good grade. This exacerbates the scarcity problem I described above and results in even more waste and an even higher pressure to impose efficiency mechanisms.
The policy of preventing students from retaking courses they already passed is one obvious answer to these problems. I agree with your analysis and with the conclusion that it is not an ideal rule, since there are situations when a student might genuinely be interested in learning a topic better by retaking a course; but in a world with a finite amount of resources in which a student taking a course affects other students’ ability to take other courses, the rule makes some sense and does not sound so unreasonable in my opinion.
I don't think that students should be allowed to retake courses to get better grades. This motivates a lazy attitude from the start, saying "well, doesn't matter, I can simply retake it". However, students are supposed to do their best.
Apart from that, I see two points that might exclude someone from retaking a course:
- Spaces available. If the course is full, priority should be given to students taking it for the first time.
- Money available. Depending on the system in your university, you (or the government, your scholarship, etc.) either pay for each course or just a general fee. In the later case, the university would run into financial problems if too many students retook the course, as the available money (for TAs, material, etc.) is calculated with the number of people taking the course for the first time in mind.
However, I can understand why you would like to retake some courses and I think it is possible, as long as you pay attention to these two points. Discuss it with the professor in charge, point out that (if necessary), you will not participate in the final exam or not hand in homework, as to not take time from the TAs. From my experience, most professors don't mind if you just sit in the lecture and pay attention, as long as you don't cause more work and as long as spaces are available. But that depends on the professor and on the university in question, so you should ask directly.