I'd guess (almost) everyone wishes everyone else wrote in plain language. But there's potentially a more fundamental reason people don't. bambax in another comment pointed out the power of ambiguity, and that's one of the reasons especially in legal / government, but there's another.

Language, specifically jargon, is an expression of culture. It's an expression of power, education, experience, or lack thereof. It's a quick way to gauge if someone is "like you" or not. If they have the same experience, and especially in professional settings, a quick (yet often incorrect) measure of competence.

Take everyone's favorite, "Business English":

You don't try again, you iterate. You don't take another call, you jump or hop to another call. You sync, and you align. Against the article's recommendation, you nominalize all day, by "bringing projects to completion" and "moving forward with the engagement".

Jargon and style, while often utter bullshit, is a way of measuring people quickly. I once had someone ask me, "would you say you practice disruptive innovation methodology?" My answer to that question was a quick way to learn that unlike them, I did not have an MBA. Complexity is often synonymous with education.

> Jargon is a type of language that is only understood by a particular group of people. You can use jargon when writing to people who will understand the terms and phrases. It can be a useful form of shorthand. But try to avoid using specialist jargon on the general public.

Unless your goal is to identify if someone is part of said particular group. And therefore in the vast majority of business and technical cases, Plain English -- at least jargon and nominalization -- unfortunately may close more doors than it opens.