Because it's not only the Bible that uses historic vernacular.
Take for instance the Romans and how they phrased quotations on engravings underneath their monuments or busts, and then think about if we had teachers rephrase these quotations so that we understood them in modern context. Why wouldn't we apply this to other writings?
Now, if we argue that we can't expect secular teachers to teach us the changes within our Bible, fine. But, surely, this is where the church comes in?
For example, our church uses the NIV. Seemingly, even some of the words and phrases which I consider to be 'everyday' and basic, are beyond the scope of what some have been taught or come across often, so our pastor provides alternate words or summarizes phrases in a more 'modern' way for generalized understanding.
The responsibility for learning has to lie somewhere. We can't all just delegate responsibility to someone else or just cease the pursuit of knowledge. There are many paths to learning if someone is willing to pursue them.
I don't wish for this to sound flippant so apologies if it has come across that way.
My point here, I guess, is I learned Shakespeare in school, I learned about Plato, I learned geometry and about Magellan and governments of countries I've never even heard of. None of my knowledge regarding how people spoke, learned or wrote at the time their ideas were formulated was inherent, some of my knowledge changed from how it was presented to me initially etc and if I had curiosity about them at a later date, due to a passing conversation or something I read, I sought to update what I thought I knew or reconfirm what I'd already learned.
I have taken the point of this post to be that there is no reason for us to speak in the colloquial ways of the KJV and it is not understood why anyone would choose to do so since no one really understands what is being said, in the main. However, I don't see that as true across the board and there are reasons why the KJV is referred to by many despite the language being outdated.
I also think that if a church is using the KJV, then it's okay to ask them about what certain things mean if we don't understand since we don't always know how to apply KJV text to modern language. They would have, in my opinion, a responsibility to clarify but we would have the responsibility to ask so we can learn.
I can't imagine being bothered by someone learning, teaching and applying physics just because I didn't understand it or the origins of it. If I felt that strongly about my lack of understanding, it would seem to me that it's my responsibility to learn so that I do understand or learn enough so that I can ask questions from the appropriate source.
Apologies if I have misunderstood the post!