It could be different explanations for different people. This said, options 1 and 2 seem to contradict with my experience that students object even against explaining relatively simple non-mathy things. My experience comes mostly from high school where I taught everything during the lessons, no homeword, no home study; this seems to rule out option 3.

Option 4 seems plausible, I just feel it is not the full explanation, it's more like a collective cooperation against something that most students already dislike individually.