Even if you do get paid as a Ph.D. student and/or a postdoc, more often than not the stipend that they get is still a small amount of money to live on. Lots of folks still have to eat/provide for their families, and that money has to come from somewhere, whether it's family or other sources.

College athletes get tuition remission, too, but there were cases of students going hungry and not being able to take care of themselves - that's why the NCAA allowing students athletes to make money from their likenesses is a BFD. I suspect grad students fall into this category, too. Tuition remission means jack shit when you can't afford to live.

People just don't go on strike for funsies.