When Juliet asks "wherefore art thou Romeo?" she knows exactly where he is.
Sadly not true. He's in the scene, but spying on her so far. She does address him later in the scene:

Jul. How cam'st thou hither, tell me, and wherefore?
The orchard walls are high and hard to climb,
And the place death, considering who thou art,
If any of my kinsmen find thee here.

This could make sense if 'wherefore' meant "where from". But that would make all the other uses even less sensible than "where" makes them. And we have a word for "where from": 'whence'.

The second one is that, if you know the story, then the verse itself is a bit confusing. The problem isn't that he's Romeo, it's that he's Montague.
She sort of clarifies a bit later:

Jul. O Romeo, Romeo! wherefore art thou Romeo?
Deny thy father and refuse thy name!
Or, if thou wilt not, be but sworn my love,
And I'll no longer be a Capulet.

Jul. 'Tis but thy name that is my enemy.
Thou art thyself, though not a Montague.
What's Montague? it is nor hand, nor foot,
Nor arm, nor face, nor any other part
Belonging to a man. O, be some other name!
What's in a name? That which we call a rose
By any other name would smell as sweet.
So Romeo would, were he not Romeo call'd

Granted a confusing clarification, implicating both 'Montague' and 'Romeo'.