First, the grad student/professor relationship is inherently asymmetrical, much more so than most employer/employee relationships. The degree has to last years or else you get nothing, and there are no hard and fast terms of the relationship like there is in many jobs. The professors recommendation might be the most important thing you get, and that is always running in the background. So many professors treat their students like robots.
Second, professors are essentially super students. We became professors because we have extreme standards. Your engineering manager became a manager because he was an engineer and got promoted, but having a professor as a boss is like if a famous open source developer was your boss. Nothing about becoming a professor selects for kindness or empathy, unlike a manager at an ordinary company who might be trained and evaluated—-especially in a large company—-on the happiness of his workers. So a lot of professors have essentially obsessive attitudes and no empathy. Add to this the fact that many students are escaping countries where they have little future. For example, the past few years, the Iranian economic crisis has caused a surge in Iranian applicants.
Third, the professors themselves are under extreme pressure. Everybody knows this.
Fourth, the work a professor outsources to students tends to be the stupidest and worst tasks, because the students usually aren’t good enough to make big contributions. This is depressing for the students: they’re smart people who signed up to work with a famous professor on work they cared about, but their day to day is to do things anyone could do for almost no money.
Finally, the professors are under pressure to create these huge labs. We don’t make profits, so things like funding and number of PhD students are taken to be metrics for evaluating. The US news ranking absurdly includes phd’s per tenure track professor as one of their metrics. So a lot of people are accepted who really shouldn’t be there, and the research is designed to have tons of busy work.