Perhaps I am generalizing a bit here, but (at least in the online education world) many of the science and engineering MOOCs teach students how to understand and use particular equations and solve certain kinds of problems based on that knowledge. Students are tested by completing exams and problem sets. These are discrete skills, for discrete tasks, and the questions usually have one correct answer. As mentioned, MOOC platform tools are largely built around this model.
In contrast, for social science and humanities classes, it is not only about teaching facts or getting students to "the right answer." Teachers challenge students to think critically, sometimes questioning knowledge that is presently held as self-evident or taken for granted. The experience can be pretty jarring.
Therefore, in traditional real-world (i.e. non-MOOC) classrooms, the pedagogical form is often distinct from the one employed in STEM lectures. The favored model for political science or literature classes, for example, is the seminar format, with the emphasis on discussion-based learning. A student does not simply go off on his own, read the textbook to acquire factual knowledge, and complete an exam by solving for the correct answers. Lectures are certainly used in political science, but these are supplemented by discussion section.
It is not the case that every student always participates in the conversation; just ask the next frustrated PhD who is TA-ing a class about how silent the room sometimes seems. But even active listening to others engaging in discussion helps students think through the material, as they may privately raise questions and objections. It is not a lone-wolf model of learning that you can get away with in a math or chemistry class. Being forced to construct arguments based on evidence, defend them, and hear multiple perspectives on the same issue, are all useful means of interrogating new concepts introduced in class. There is, of course, room for reflection and "alone time" and literature and political science students are just as often found with nose in book when first reading the material. But the social nature of classes, modeling the interaction of ideas in an engaged debate, after the reading is completed, is recognized as a key part of the learning process.
If you're asking people to think differently, it's simply less effective to learn in a vacuum. Some STEM classes do fit in this interactive realm as well -- for instance, courses on sustainability often benefit from discussions and asking students to consider how issues inter-relate. Perhaps it's because for many environmental problems, there is also no "one right answer." We are asking students to absorb facts, comprehend the complexity of issues, and reframe those issues while understanding that there may be multiple solutions.
I'm not saying one is better or worse, but simply noting that different approaches are required for different kinds of learning. We make this distinction in real life classrooms, and we need to make the distinction in online learning as well.
P.S. I recognize that in actual science and engineering classrooms, educators do aim to teach students to be critical thinkers. The goal is usually described as having students comprehend the underlying principles so they can apply them in diverse contexts, be these engineering scenarios, biological mysteries, or a mathematical challenge where computational power can be brought to bear. But in MOOCs especially, much of the focus is on discrete, testable skill sets. These skills can be assessed, and students presumably demonstrate gains in scores as they learn the material. (While this issue prevalent in MOOCs, it is hardly limited to them. Sit through a run-of-the-mill freshman math lecture for another example).