Grades seem to be the primary focus of students who judge their performance by a little letter that gets noted in their transcripts. They put pressure on professors to raise those grades but don't often consider the greater significance of learning. The end result is rising grades that have little impact on learning.
There are a couple of factors that are at play that create this situation. They are 1.) the student, 2.) the professor and 3.) the university system. When one of these factors are not in alignment grades begin to inch upwards thereby reducing the quality of education.
The student has a natural interest in getting the highest grade with the least amount of work. It is a natural effort and reward calculation.Asking students to think about the long-term consequences of high grades without knowledge mastery won't have much effect because many don't have this level of awareness.
Some professors simply don't care and don't want to have poor ratings by students. By giving their students high grades they get less questions, have less complaints, and in turn have less work to do. It is easier to grade lightly and avoid time consuming challenges.
The university system
Administration determines the performance networks that reward/punish professors for their work. If student complaints create havoc on professors sense of security, grade appeals are constant, and the metrics reward those professors who don't challenge their students, then most professors will get lenient.