Because students wouldn't do the homework and would fail the quizzes. Students need to be pressured into learning and grades for doing the practice are a way. Don't pretend many students are self-motivated enough to follow the lecturer's instructions when there's no grade in it and insisting that "trust me, you won't learn if you don't do it".
I've mostly had non-graded homework in my studies because cheating was always easy. In highschool they might have told your parents if you don't do homework. In university you do what you want. It's never been an issue overall.
Well, from what I understand, the answer is kinda "no".
Depends on the country and educational system I suppose, but I do believe professors in many places get in trouble for failing too many students. It's right there in the phrasing.
If most students pass and some fail, that's fine. Revenue comes in, graduates are produced, the university is happy.
If most students fail, revenue goes down, less students might sign up, less graduate, the university is unhappy.
It's a tragedy of the commons situation, because some professors will be happy to pass the majority of students regardless of merit. Then the professors that don't become the problem, there's something wrong with them.
Likewise, if most universities are easy and some are really hard, they might not attract students. The US has this whole prestige thing going on, that I haven't seen all that much in other countries.
So if the students overall get dumber because they grow up over relying on tools, the correction mechanism is not that they have to work harder once the exam approaches. It's that the exam gets easier.
For the most part degrees from roughly comparable schools in the same subject are fungible. However, graduating cheaters who should have flunked out of school their freshman year is a one-way ticket to having a reputation that your degree is worthless. You're now comparable to a lower tier of schools and suddenly Y's degree is worth a lot more than yours. The best way (not to only way) to combat this is to actively cull the bottom of your classes. Most schools already do this by kicking out people with low enough GPAs, academic probation, etc. My undergrad would expel you if you had a GPA below 1.8 after your first semester, and you were on academic probation if your GPA was > 1.8 and <=2.5.
This assumes, of course, an institution is actively trying to raise the academic bar of its student population. Most schools are emphatically not trying to do this and are focused more on just increasing enrollment, getting more tax dollars, and hiring more administrators.
Many mathematics professors don't require homework to be turned in for grading. For example, the calculus courses at many US universities. Grades are solely determined by quizzes in the discussion section and by exams. Failure rates are above 30%, but that's accepted.
This model won't work for subjects that rely on students writing reports. But yes, universities frequently accept that failure rates for some courses will be high, especially for engineering and the sciences.
When I was a student, I spent my first 2 years in a so-called prépa intégré of a French engineering school. 20% of students failed and were shown the door during those two years (some failed, some figured that it just wasn't for them). That's fine, that means you keep the ones who actually do the work.
At a certain point, you have to start treating students like adult, either they succeed or they don't but it's their personal responsibility.
My favorite math professor said "your homework is as many of the odd-numbered problems as you feel like you need to do to understand the material" and set a five minute quiz at the start of each lecture which counted as the homework grade. I can't speak for the other students, but I did more homework in his classes than any of the other math classes I took.
