Part of that is because Chicago is considered the baseline "accent" for many news broadcasters, radio personalities, etc. As a result, most Americans have heard that accent their entire life and therefore don't really interpret it as all that different.

I've found that it's the keywords that indicate where people are from within the Midwest:

Is it soda or pop? Is it car-mel or care-uh-mel?

(I do linguistic stuff in my day job.)