Comment by LetThereBeLight

A large issue is that PIs have no training on how to manage people nor is there much protection for grad students (and practically none for postdocs). It is surprisingly common to hear stories of sexual harassment, or international researchers essentially being held captive by their advisors. The universities are motivated to maintain their public appearance, and so cases of harassment and mistreatment are either entirely ignored or dealt with quietly. This, in addition to pay issues, is also playing a significant role in the unionization efforts that are taking place across US universities.