Why can’t people just say things in plain English?

I’m glad you asked, because there are several good reasons, not the least of which is that no English sentence is equally plain to all native speakers of English.  This is because of the wide diversity of regional and cultural dialects in the language, and the equally diverse personal experiences that make up our knowledge of the world.  Throw in the typical ambiguity inherent in popular definitions and usages of English words, and you are already at a disadvantage before you even get started.

But even beyond the vagaries of the language itself, there are very good reasons to state things in ways which might leave the average person exasperated.  The most obvious example is legal language, which also happens to be the most often disparaged.  This is a context in which ambiguity can cause real and serious problems. The solution is carefully and laboriously defined terms which evolved through a long process of eliminating alternative meanings. Obviously, this sort of thing can be used to confuse as well as to enlighten, but it’s main goal is to forestall as many misinterpretations as possible.

There are plenty of examples of obfuscation without resort to fancy language, by the way.  I need only point to the President Who Shall Not Be Named.  As a matter of fact, it is exactly his kind on confusion that legal jargon attempts to avoid.

Or, put another way, jargon is nothing more than saying exactly what you want to say, and only exactly what you want to say.