Having compared three different schools that I've worked at, I think I know why.
A full-time teaching instructor is expected to teach four classes at a time. From my experience, teaching a class takes about ten hours a week (this includes about 4 hours in class, plus extra time for prep work and grading). So far, that's a 40-hour work week, which is expected.
But then you have office hours, typically anywhere from 4 to 10 additional hours per week. If your students don't show up then you can use this time for grading, but it seems to me that if you're counting on your students never visiting you then that's a greater problem... but it's certainly not something you should be encouraging as a teacher.
Then there's academic advising, which is nothing most of the time but makes for a week of hell somewhere near the end of each class, as you get a flood of students with paperwork. So far I haven't been involved in this process enough to say what the time commitment is, but I think I can reasonably say that it's not zero.
There are department meetings, which is an extra hour or two every week or two. Already we're somewhere around 50 hours per week. If you want to do anything extracurricular, such as be the sponsor of a student club or perform community outreach to local high schools or offer seminars to your faculty colleagues or what have you, that's extra. Some schools mandate that you put in a minimum number of these kinds of extra hours, others don't.
Now, those of you in the game industry are scoffing at me here. I'm concerned about a mere 50 hours, when it's not unheard of in game development to pull 90+ for extended periods? Ah, but here's the rub: game developers are, by and large, a passionate bunch. We got into the game industry specifically because we love games and want to make them. I'd say that of the professional developers I've worked with, somewhere around 90% of them have a passion for their work and are more than willing to put some extra time in if it'll improve their project, or if it'll give them a chance to improve their own skills and hone their craft.
Teaching is different. Of all the professors I've met, maybe 5% are passionate about teaching, so very few are going to willingly put in the extra time unless forced at gunpoint. And the thing is, with both teaching and game dev, the quality of the final product is proportional to the amount of work you put in.
There are other things that modify a teacher's workload:
- Studio/practical classes take significantly less time than lectures. You just have to design assignments, so the amount of prep work before class is minimal. Strangely, it counts as the same, so loading up on studio classes is a way to game the system. Of course, someone has to teach the lecture-based classes, so you're just reducing your workload at the expense of your colleagues (who are probably not as passionate about teaching as you).
- Online classes are insidious. They seem like they should take less time because there is no lecture, but I think they actually take slightly more time because you have to log in, check email and contribute to discussions on an almost-daily basis. Think about how much time you spend just checking your email, RSS feeds and discussion forums in the morning and you'll see what I mean. The time flies by so it doesn't feel that bad, but the total time per week is a little more than a typical lecture class, so you have to be careful. Some schools recognize this and actually pay a slight premium to online instructors; others treat online as equivalent to a "normal" class.
- For lecture classes, my above figure of 10 hours per week depends on two major factors: amount/intensity of grading, and amount of previous course prep. If your assignments are easier to grade (e.g. multiple-choice as opposed to essay questions) that will reduce your time commitment, at the expense of having assignments that are meaningful -- the Real World rarely gives you multiple choices, after all. As for course prep, a class requires more time the first time you teach it. Some schools give you a break if one of your courses is brand-new and developed by you (say, only teaching 3 classes instead of 4), while others make no such allowances.
There's a common theme here. Almost everything that a teacher can do to make their own life easier, does so only at the cost to the quality of their students' education. Which means that the teachers who are passionate about teaching and really care are the ones who spend 60+ hours per week, and everyone else is going to do whatever they can to bring their hours worked as close to 40 as possible.