There are definitely different types of graduate degrees out there. A professional-oriented degree (i.e. law, business, medicine, etc.) is intended primarily to teach you the latest body of knowledge and prepare you to work in a specific profession (or enhance your qualifications for one). There wouldn't be much point in spending your time working on original research (i.e. a thesis or dissertation), because that's not what the degree is for.
A research-oriented degree, on the other hand, is intended to teach you how to contribute original research to the field in question. The only way to prove that you can do it is to publish original research. Coursework to learn about the latest body of knowledge in your field is required, but usually only at the beginning of the degree program.
Of course, those same graduate students in a research-oriented degree do have their knowledge of the field tested quite rigorously before they are free to devote their time to their thesis or dissertation. If you've ever heard anyone talk about their "comps," it's basically the grad student standing up in front of a panel of professors and answering as many questions as the panel can throw at them. When the panel is satisfied that the grad student is sufficiently knowledgeable, they are free to research and write their original contribution without taking additional coursework. I think my husband read something like 200 books related to his field of study in preparation for his comps.
Defending a thesis or dissertation is kind of the same idea (you get asked as many questions as a panel of professors can think of), except that it's your work that's being questioned.
Fortunately for the beleaguered grad students in the world, no advisor worth his/her salt would allow a student defend before that student is able to do so successfully. I've heard my husband say that, by the time you get to your defense, it really ought to be a big old love fest (because you've already addressed all the potential issues in the editing you did prior to submitting your dissertation for your defense).