Why is the article before "Wall Street" extraneous?
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Because the name of the street is "Wall Street" not "The Wall Street." I've seen the sign, trust me. :)
Seriously, though, the only time you would use the definite article before a street name would be to distinguish it from other streets of the same name. If there were a Wall Street in Podunk, Iowa, and you were wondering if I was talking about that street when I mentioned Wall Street in a conversation, I might say, "No, I mean the Wall Street." You would understand that I meant the famous Wall Street in Manhattan as opposed to the (obviously inferior) Podunk version.
I suspect this may be a more subtle question than it appears: why do proper nouns not take an article in English when they do in other languages? In French, for example, you would say "The Captain Smith drove down to the Wall Street."