It is for the same reason that anyone ever nudges anyone else about pronunciation, spelling, or grammar: speed of communication.
If you pronounce "/etc" as "et cetera", then it will add a step in me parsing your speech ("Oh, he means 'etsy'").
Second, now I have an awkward decision to make when speaking back to you. Do I adapt my pronunciation to yours for the length of this conversation? Or do I pronounce it my way, causing the extra step in your translation of my speech, then also causing you to wonder why I am saying it differently, furthermore causing you to consider whether to change your pronunciation to mine, if nothing else to end this extra thought process every time?
Also, if I adapt my pronunciation to yours, I guess now I need to remember to say it your way every time we talk forever, and say it Henry's way when speaking to him, and now I have this whole table in my head of various pronunciations for various individuals.
And then what happens if I am in conversation with you and Henry at the same time? Do I say things your way, his way, or my way? And what do you do? And what does Henry do?
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Or we could just stop, agree upon a common pronunciation, and move forward at the normal speed of conversation.
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To clarify, the tone never should be condescending. This is a small matter. Therefore the tone should be light.
The pronunciation that is deemed "correct" is usually the one of the majority. So it is just an attempt to get everyone on the same page. It strikes me more as "inclusive" than "anti-inclusive". If I were "anti-inclusive", then I would silently let you say it your way, smile inside, and then talk behind your back to Henry about what a newb you are.