This is despite hundreds of years of effort by well-meaning, perceptive, and hard-working teachers and students to refine the presentation to make the subject as clear as possible. Because most teachers and most texts are not the dipshits of the world. In short, Calculus is hard.
The thing about massive courses is that they are massive. When you get big, you cannot help but get average. In my experience average students (say at the schools I mentioned above) need support, including reasonably intensive personal attention from an experienced and knowledgable teacher. I don't believe the typical person in my course can pick it up on their own. (Not to dismiss the potential; I think there is a lot of exciting stuff happening that can help people learn. But I've seen nothing that would make attention unneeded.)
I do perceive that many HN readers could just pick Calc up, and frankly so could I, which is a good reason to read HN. People here pique my interest every day. But often in the discussions following articles like the one here missing is an understanding that you (speaking to a typical reader) are not average.