Lack of motivation for self-study during the semesters
I do think that your perception is shared by many students, especially when they are passionate about their field of study. I can think of several undergraduate students (former colleagues or under my supervision) who fit your description, across several countries. I would not be surprised to learn that this perception is much less frequent among graduate students, because curricula and self-interests tend to align with each other over time.
At the undergraduate level, self-motivation is wholesome and certainly improves your chances for academic success. Nevertheless, curricula are designed by professors who do know best.
Even when a class seems dull or irrelevant, the decision to include it in the academic requirements for your degree is the result of a consensus selection made by your professors. They are in a better position than you are to weight-in certain factors, such as the difficulty of the material or its usefulness as a foundation for more advanced and exciting stuff.
TL;DR: Not many people enjoy studying pendulum problems. Thankfully, some trust their instructor at this stage and access broader horizons as a reward.
Not finding the time or peace of mind necessary for self-studying is a sign that the expectations of your curriculum are catching up with your work capacity. Then you are making the right choice of prioritizing the mandatory coursework.