Why Is It That The People Most Interested In "proper English" Often Know The Least About English?

A friend of a friend wrote a post (beginning with "BTW") about he's a stickler for "proper English". I wrote the following response:

Just an FYI (which has had recorded use as an acronym since the 1940s), the term "proper English" is a misnomer. All languages have a variety of dialects and colloquial forms. Valuing one dialect or sets of colloquialisms over another, it seems to me, is either a form of classism or regionalism. 

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Take for example the use of the word LITERALLY. The Oxford English Dictionary (either the authority on the English language or a pedantically conservative institution that lacks the agility to comprehend the evolution of a living language) acknowledges--despite its own balking--that there are definitions for the word that are at odds.

I. 1. a. In a literal, exact, or actual sense; not figuratively, allegorically, etc.

I. 1. c. colloq. Used to indicate that some (freq. conventional) metaphorical or hyperbolical expression is to be taken in the strongest admissible sense: ‘virtually, as good as’; (also) ‘completely, utterly, absolutely’.

Now one of the most common uses, although often considered irregular in standard English since it reverses the original sense of literally (‘not figuratively or metaphorically’).

While some may argue that the protests and embedded judgement of our new societal understanding of the word shows that this new definition is improper, it should be noted that the offending definition is still given space and is accepted as a definition.

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Another excellent example of the language evolving is the word BI-MONTHLY. Having been incorrectly used instead of SEMI-MONTHLY so consistently, we now--as a people--accept both usages.

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Parenthetically, I should point out for those not normally interested in grammar that the use of BTW is, itself, a colloquialism. 

It was not widely used until the 1990s--the dawn of what many consider the death knell of the English language. Before then, its use as an acronym would have been considered "improper English" used only by troglodytes. (It should go without saying that present company is, of course, excluded.)

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In closing, do not take my word for it. In this entire post, I have only used one space after each period (a practice unheard of before the 20th century and deemed archaic in the age of the word processor); I have used my commas in a variety of informal and ignorant ways that do not conform to the Chicago Manual of Style (each erratum, no doubt, causing my 10th grade English teacher to cringe); and I am a grown man who has spent the day watching superhero movie trailers on YouTube. ;)

2018.10.02 Addendum https://en.wikipedia.org/wiki/Muphry%27s_law