alt Hacker Newsacademic writing is designed so a paper is part of a conversation, i.e. 100 other papers strongly relevant to the current paper. And the author needs to compress the ideas from those 100 other papers, plus their own additions to the conversation, into 6 pages.
Keep in mind those 100 other papers also went through this kind of data compression.
So the number of ideas/concepts per paragraph is much higher than 'popular' writing, and some base familiarity with the concepts under discussion needs to be assumed.
Yes, it is hard work to read these. Even when you are active in the field. Generally I need to read at least the abstracts of a some of the key references in order to understand the paper I'm interested in.
Information density is one part, precision is another. Papers are often presenting work at the frontier of the field, which is by nature not well understood yet, and competitive. To have something worthy of publication is to have something that is new, and that often requires a degree of precision to communicate that we don't use casually. I think it's pretty gross to denigrate "academic writing" as obfuscatory, just like it's gross to make broad sweeping generalizations about journalists.