Could well be, and I have no definitive evidence otherwise, but sometimes it's hard to imagine anyone being that bad at exposition. Some "benefits" might be seeming smarter and more knowledgeable than they are, making it harder to exactly replicate their methods (most relevant in applied areas), or citations by intimidation ("I don't understand this, but it seems impressive and relevant, so I better cite it.").
They all sound plausible and likely. There are people obfuscating knowledge for all the reasons you mentioned.
At the same time, it doesn't mean there's a concerted effort to obfuscate mathematics.
Explaining things is hard, because you have to create a clear picture of the audience in your mind. Then, you have to draw the lines between what they know and what you want to introduce. The curse of knowledge is a big obstacle here.
