The word used in Romeo & Juliet is actually "wherefore" but you're right, that specific word is actually saying "why are you" not where are you. Juliet is basically asking "why are you Romeo and a Montague, my father's enemy?" However, the question in this topic is simply "where",
not "wherefore" (wherefore is "why", "where" is referring to location), so in that sense, the question posed "where art thou" is correctly translated to "where are you?"
Yes I know, off topic....another Shakespeare geek here.