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

it was illegal for private schools to have grad student unions until ~2016, as (private) graduate students were not classified as employees.

https://www.aaup.org/brief/columbia-university-364-nlrb-no-9...

Yeah, private universities being unionized is more recent.