Plain English should not be confused with low literacy level. Plain English is a style that is hard to master. Only 12% of U.S. adults reach the highest PIAAC literacy proficiency levels.
"unnecessary verbosity" is just one way to express your position in the society. Wearing impractical clothes, complex etiquette, having pale skin and speaking in certain manner was something that requires either wealth or practise.
Today business jargon combined with clothes and behaviour still work today as signalling your identity. So is wearing hoodie and saying "bro" constantly. People learn to feel comfortable in the uniform and in the language they identify with.