Hard subjects and lazy students. I honestly understand it, because not learning these things sets you up for failure in future courses and exams are the only way to force students to actually dive into it.
Still, it's quite a shock coming from a more business/communications-oriented bachelor where I scored pretty much straight As and graduated top of my class with honors. I'm really not used to German engineering degrees and actually failing classes. I mean, in most of these degrees, like 30% of all freshmen actually finish the thing.