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Why is it that people think "How come" is an acceptable use of the English language?

Because it's been a perfectly valid, universally recognized synonym for "why" for at least two centuries, and much longer than that if you include variations such as "how comes it". The real question is, where did you get the idea that the phrase "how come" is unacceptable? Is it in any way unclear what the poster was asking?
 
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