To make "next to nothing" concrete: MIT EECS PhD students are currently paid about $4700/mo. This is substantially less than they'd make in industry, but it's around the US median personal income across all working-age adults, and well above the average 24 year-old. They frequently make a substantial extra at summer internships, putting them well above US median in the years when they do.

Also: it is school, not just a job. They are developing deep expertise and specialized skills. As a result, among other things, their earning potential tends to be significantly higher coming out of the PhD than out of undergrad.