The reason this happens is because students care more about grades than learning. It wasn't always this way. I wish advisors would simply tell them that at this level, learning is more important. We're training kids to think that only immediate assessments matter (grades, tests) whereas in the past, learning was considered a more holistic experience.
Most of the professors simply do not have the same experience with assessment and grading, so they put a lot less stock in it. Which makes them more willing to avoid confrontation.
Plus, at most universities, 1/4th of every class is going to be stocked with kids that don't put forth much effort. They want Cs, they get Cs and Ds. What I see in my position at a university is a lot of A-s, As, a smattering of Cs and Ds. The most forgotten grade of all is the B-range. When I was "trained" to teach back in the early 90s, the people overseeing us would throw a fit if our class's GPA was not within .15 of the supposed target of 2.7 (a B-). That too was a bogus arbitrary standard.
After grading kids with Bs (an average of 3.0s) for several years, I was once brought into my supervisor's office and I was told that my entire portfolio of publication, service, awards was going to sail me through the tenure process, but I was given a talking to about my teaching evaluations, was told that this would be a hangup for some on the college's appointment committee. They asked if I needed help and to think about what I could to improve things in the 2 years I had until tenure review. So, I experimented. I jumped my averages from 3.0 to 3.3 or 3.5. My evaluations skyrocketed. When I met with my supervisor the next year, he asked what I had done to jack up my evaluations to the top of the department. I answered: "I'm giving them higher grades." He gave me one of those pointed finger "right-on" gestures and nodded, yes. What does that tell you?
There are legislators in Texas who want to make teachers accountable to the customer (students) so that teachers will be rewarded with higher salaries if they have stellar evaluations. If they have poor evaluations, their salary drops. This is a real proposal.
Just imagine if the NFL worked this way. Imagine if the Terry Glenns and Adalius Thomases of the world could determine the worth of the coaches.