Everyone is different, but in my case it's because:
-We take a lot of classes. It is the longest degree plan at my school. It is very fatiguing. I need 140 credit hours for the bachelor's degree, and I will have 160 when I graduate.
- The classes we take are some of the hardest/most rigorous in existence at the undergraduate level.
- Our professors greatly vary in quality. Some are total dicks that only care about their research, thus they suck at teaching. Others are good but very hard.
- The things my classmates and I have had to put up with have broken us into calloused, cynical students.
Saturday night finals? Sure, why not, the professor wants to do that.
Completely lying about content that will be on the exams? Sure, why not, if it gets you to study for days on end because you can't trust your professors.
Four years of courses that are all prerequisites to each other and only offered specific semesters? Yeah, because we don't care about you graduating on time.
Exams the day before Thanksgiving? Yeah, we didn't want you to be with your families anyway. Be sure to turn in that 77-page technical report the Monday you get back!
Undergraduate final exams that consist of only 2 questions and the average is a 40/100? Yep!
Software exams on computer software that crashes and loses all of your work in the middle of a test? Yep.
Homework assignments for one class that take 20 hours a week? Par for the course.
Professors who are only available for help 1-2 hours out of the whole week, often during other classes? Pretty standard.
The halls of the ChemE building at my school are populated with audible moans from the students every day and a white board where the senior class marks down the days until graduation. We all hate it; even if you like chemical engineering, you hate the workload.