I think maybe it depends on who say that and how his expression.  I

I'm Indonesian, also have dark skin, well not really dark but not also white like western people. And if someone asks me that question, I will answer "Because I live in a tropical country which the sun shines so bright here. That's why"

But I don't think that question is racist, maybe they're only curious why we have dark skin. Just answer them with positive thinking

Ah, Jessy, I dont like the sun. I am white. People are constantly asking me why are you so pale? I dont consider that racist. I consider it exasperating though.

    I am unsure if I would conclude the remark to be "racist"; but I can tell you for certain Jessey, that the question is utterly Stupid, if not entirely inappropriate.

 If someone  who asked you this was a chat partner on Skype,   I would remove such a person from my contact list in a hurry.

 I would never ask a person   a question like that.

  There are "idiots" in the world Jessey. Avoid them whenever you can. Your life will be so much easier for it.    All Skin is fine,  just the way it is.  I don't need to demand that somebody explain their "skin" to me.