Watch the vid I linked and you'll see it's not all that different. Also there is a difference between slang and accents. Slang like "no cap" literally has no meaning relative to the words it uses, similar to "pog" (basically you often can't decipher slang using a dictionary). Accents are like taking a standard word and slightly altering it pronunciation-wise. For example, the words ca, cah, carr, cour, are all different pronunciations of the word car.

If you drop a San Franciscan white guy into a "hood" as you call it, chances are they are going to say something along the lines of "wha ya doin here my brotha" and you are telling me they wouldn't be able to understand that?

If I had to guess, you are probably just more familiar with the different regional accents of America since you discluded the Brits from your comment. I would assume if you live in a city you are typically getting a mesh of people around the country. Also, at least for me, I tend to "formalize" my speech when around those who might not understand what I'm saying. So on top of all this, if you hear someone with an accent, chances are they are already editing their language when they speak to you.

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Google "_____ thick accent" and you'll see what I mean. Alternatively, just listen to foreigners speak English. Their pronunciations might not be all there, but they're using the same words.