However, after letting off all that steam a question popped up in my head: "From the teacher's viewpoint, what is the goal and function of compulsory pre-exam exercises?"
And that changed my whole outlook on the exercises. In my experience, at a research university, which offers also bachelor's degrees, when dealing with a compulsory course attended by 150-300 undergraduates per year, the teacher has two main goals:
1) that arranging and teaching the course does not take too much time from the teacher's own or their postgraduate students' (= course TAs) research
2) that as many students as possible pass the exam on their first try = exam checking does not become a too depressing task for the teacher and the TAs
From the viewpoint of these goals, the role of computerized, compulsory pre-exam exercises becomes quite clear: the purpose of those exercises is to make sure that each student has - at least once - successfully worked through each of the basic problems that they are likely to encounter in the exam, i.e. that each student who comes to the exam has at least a minimum level of knowledge and thus a fighting chance to pass.
So the exercises were never intended to be challenging: they were intended as a low-water mark, to check that everyone passes a minimum standard. My bad.
And now I feel like an idiot, again. Thankfully, I'm quite used to that
