Why do researchers sometimes use extremely complicated English sentences to convey their meaning?
I've run into variations of this a lot in economics, and have a few observations.


Being good at research in your field is an entirely separate skill from being able to communicate ideas clearly.  Two Nobel laureates really embody the extremes here for me, both with brilliant research.  One the one side is Paul Krugman, who, regardless of what you think of his public persona, is an excellent writer.  His skill comes across even in his research papers, and even in his work from the '70s.  On the other hand is Amartya Sen, who it often seems to me can't express an idea clearly in one sentence if it could equally well be conveyed in fifteen.  Granted, Sen wasn't born in an English speaking country, but he left for school in England in 1953, so he's hardly new at this.
There appears to be some institutional momentum.  That is, you learn to be a researcher in your field by reading the work of past researchers.  That frames your concept of what quality writing in your field looks like, which you then adopt to some degree.
It can be a signaling method. This is related to the previous point.  For professionals used to reading papers written with heavy use of jargon or unnecessarily-big words, writing this way can make your work seem like it belongs.  I often feel like the use of latin phrases really falls into this category.  I myself enjoy the occasional ceteris paribus, for example, but it really isn't a very defensible practice over writing all else equal.  It isn't even shorter!


I think many of us could benefit from thinking about writing more clearly, but it also makes sense to know your audience and write for them.