“A big problem is that the writing style tends to be overly passive, overly wordy and indirect.”
As a person who does a lot of scientific writing, there are a lot of factors that contribute to this:
As others have already mentioned, scientific writing demands precision and detail. You have to say exactly what you did and how you did it, and exactly what the findings support, which usually requires a lot of words. Clarity and concision are great if you can swing it, but distinctly secondary considerations.
Passive voice is useful. For one thing, the “active agent” in most papers is always the same throughout (the authors), so without it there would be an endless stream of sentences of the form “We did X. Then, we did Y. We also did Z. We observed S and T. We concluded from these findings that Q. Therefore, we next did A and B.” Just for not-being-boring-ness, it helps to be able to change now and then to “Analysis of XYZ showed that ST, suggesting that Q. Confirming this, measurements of AB…”
For another thing, passive voice lets you emphasize the important bits, and also save some words (“Slides were incubated in PBS for one hour” is both more direct and one word shorter than “We incubated the slides in PBS for one hour”).
Research papers are generally written under length constraints and usually under considerable time pressure as well. The former tends to lead to messy sentences and paragraphs, as you try to jam together a bunch of related points to save on bridging and introductory words. The latter means you don’t have time to carefully craft your deathless prose. Repeatedly passing drafts back and forth between multiple collaborators is also not an ideal way to produce fluid writing. And when the word comes down from the editors that the paper has been provisionally accepted but they want it shorter so you have ten days to cut the text by 30%, without taking any data out, and make sure all collaborators get to review the document and approve the changes before submitting… well, making it read beautifully is not a top priority.