My first term of university I felt my professor downgraded my paper only because I disagreed with his point of view. The more my writing and structure approached perfection with subsequent papers always taking a different side than held by the professor the more the papers where downgraded. I would have stayed in school if I studied a STEM field as it isn't opinionated, at that level at least. I felt that if I stayed with arbitrary grading either I agreed with and showed all professors reverence or I would not do well in school. The professors can really shape how young adults think.
I don't think that problem is exclusive to non-STEM fields. The math classes that I took as an undergrad math major had EXTREMELY arbitrary grading standards.
Some of the math classes I had to take were just plain annoying. I graduated because I put up with the crap.
Skin color or other superficial stuff can be a factor. In my experience though it's one of two reasons in the vast majority of cases: Either the work's too much for them, or they become disillusioned when they find out most universities aren't about questioning established truths and doing actual research.
Of course this isn't something you'll find brought up in many studies.
I left Stanford two classes short of a degree. I just couldn't make myself care. Difficult to explain.
Depression?
College and I came to an agreement. I agreed to leave, they agreed to not kick me out.