Which is not to say that conditions in graduate schools (or academia as a whole) are great. But the unionization process is entangled in the legal framework around unions in the United States.
It is recent and still uncommon that private universities have a grad student union. The US also has many great public universities that have had grad student unions since forever
[1] https://en.wikipedia.org/wiki/Graduate_student_employee_unio...
Which is not to say that conditions in graduate schools (or academia as a whole) are great. But the unionization process is entangled in the legal framework around unions in the United States.