Earlier quoted context omitted.
Class hours are a substantial requirement of many phd programs. You can't do independent research without knowing what's come before. :)
You learn about what's come before by reading papers and reproducing experiments. I can't imagine how a class for PhD students works - everyone has such focused research topics that how can you find something relevant to teach to more than one person at a time? When I did my PhD on optimising dynamic programming languages I would guess there were maybe ten other students in the entire world doing something vaguely si…
For political science in at least one program it was 30-36 credit hours for a master's degree. PhD was the same thing with an additional 18+ elective hours and another 30+ combined class/dissertation hours, and then the actual dissertation. Classes with few exceptions consisted of "workshops" and "seminars" over various topics (terrorism, multinational corporations, etc), which almost all involved weekly topical readings (5-7 journal articles and book selections) and weekly/frequent writings on those readings with a substantial paper for the final, sometimes with a midterm. Independent hours were also common with 1 on 1 work with one of the professors over a chosen paper topic.
As I said, discussion in class was expected, so you couldn't really wing it, and you had to be prepared or it would be obvious in front of your professor an another 5-20 students. I'm guessing this format is semi-common in the humanities/social sciences, it was for a linguistics program I looked into. Tons of reading and writing with the intent of guiding your towards a weird unexplored niche.