I've known maths PhD students who found they could only really do 2-3 hours of truly productive maths in a day, allowing them to spend the rest of the day playing games and suchlike, and after two years their supervisor told them they'd got enough interesting results to graduate.
And I've known chemistry PhD students whose supervisors felt if you're doing a mere 8 hours a day of work you might as well drop out now because you're obviously not serious about graduating, and expected their students to be in the lab tending to long-running experiments on evenings, weekends and even christmas day. And yes, we only give you three years of funding but everyone takes four years to graduate.
Adding to that, a large portion of (important) wet bench work is mind-numbing manual labor. This work really should be done by a tech, but techs are just as if not more expensive than grad students, and techs can leave the job if they're not happy. Which means that grad students are at the bottom of the totem pole of intellectual respect, and PIs who had to do the same expect a lot more "due paying".
Meanwhile, students in computational labs are working from home.
…at some labs. There’re many-many places with good PIs, decent funding, retreats, events, etc. I don’t get why people paint it as black and white.