Perhaps teachers are afraid to fail kids because they’re afraid of the parents.
My brother failed a bunch of students for cheating. He got lots of heat but stood his ground since he had absolute proof. He won in the end.
Depends on the school, the location, and school leadership. I found inner-city parents usually too disengaged to meet the teacher, so they were seldom feared. Admin insisted on passing grades, so teachers fell into line to keep up their performance ratings.
In the suburbs, parents are more aggressive in advocating for their children. It can be very unpleasant to have one of these screaming parents in conference over failing grades or cheating. If you're in a tuition-paying system, they have a lot of leverage unless admin stands up to them. They threaten to pull their tuition money and admin generally caves. Teacher gets 'talked to' or grades are revised in the system over the summer. Social promotion at its finest.