This is a very valid question, David, and it's honestly one reason why I left academia: I wanted to spread life-enhancing insights through writing in a style that the average person could make sense of. In general, though, I'd say that academic writing sounds the way it does for the following handful of reasons:

1) One is just convention and tradition. Academia as an institution is very ego-driven, even while many academics themselves are sensitive and humble. So they simply have to perform the same language that is expected of the discipline.

2) The absolutely critical elements of academic prose are clarity and precision. This is for a variety of reasons, such as replicability (i.e., people need to be able to re-attempt your study themselves in order to test it, if they so choose, and this is only possible if you are 100000% clear about your lines of logic), or for simply ensuring that the message gets interpreted with total accuracy. Conversational prose might sound clearer, but you'd be surprised how many vagaries it leaves. Erudite-sounding latinate words, by contrast, tend to have very precise meanings that conversational words don't — so the practical preference is for the "fancy" words that most people don't even dream of using in their day-to-day.

3) Because everyday language leaves so many ambiguities, and because it's professionally risky (and sometimes a threat to public welfare) if a research paper does mis-convey its conclusions, it is even more important for researchers to phrase their conclusions with care. There are lots of caveats and qualifiers that they need to be sure to include, rather than offering a straightforward write-up.

This is why academic writing (and academic editing) is such tedious work, let alone laborious reading.

I hope this helps explain it! :)