Every author has their own motivations, but I will echo what the parent has expressed that often, if not the majority of the time, authors are making their papers far more difficult to read than they need be.

To that, I'll add anecdote. Some years back, I had a fight with a coauthor on a paper who wanted to remove the majority of the substeps within a proof. He believed that it was condescending to include such material and that any interested reader should be able to derive it for themselves. And, in fact, he asked a colleague in the room at the time who agreed with him. I stated flatly that as the primary author that I could not understand nor complete these proofs without these steps, so the material stays. I contend that at least in this one particular case a combination of exclusivity and arrogance led to an attempt at obfuscation.

Remember, publishing isn't just about sharing knowledge. It's also a way to posture, advertise, build a brand, and get grant money. Beyond that, mathematicians are still people with all the imperfections that implies.

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.