"From Middle English wherfor, wherfore, hwarfore, equivalent to where- (“=what”) + for. Compare Dutch waarvoor (“what for, wherefore”), German wofür (“for what, what for, why”), Danish and Norwegian hvorfor (“wherefore, why”), Swedish varför (“wherefore, why”)."
Juliet is not asking the moon where Romeo is - she's bemoaning the fact that he is a Montague and she is a Capulet: Why did you have to be a Montague?
It drives me crazy every time I hear a performance (typically high school or amateur productions) in which Juliet asks "wherefore ART thou Romeo?" instead of the proper "wherefore art thou ROMEO?"
*sigh* The tribulations of an old English major...