A big part of the reason these unionization efforts are happening: https://csstipendrankings.org/ Many universities are not paying a living wage for the area they are located in.
If you include tuition at the rate undergrads pay it as part of the compensation then they are paid much more than the median wage. Average annual tuition alone for many research universities is higher than the median annual wage.
If tuition was not waived, PhDs would not even apply. The way the PhD is set up in the US is a little byzantine. Other countries (Canada, continental Europe) do have normal employment contracts and benefits.