Not sure if you're being facetious or not, but if not, I would respond to your question by restating, because we are vain and because we are obsessive. Because we place a high degree of importance on our appearance, and because we obsess over minutae like knot dimples and pick stitching and shoe welting methodology and a whole slew of other things that most people are basically unaware of and/or could care less about. We are dandies (or at least wannabes), we are freaks, and these are the common 'threads' that bring us together and keep us here.
No, I was being serious. Are we vain? Insecure? Obsessed? It would be an interesting topic for discussion. A sort of meta-discussion of discussion, and then we could find some more meta-minutiae to discuss. Sorry, I'm having trouble staying serious tonight. Then again, you use the words vain, obsessive, freak; but I don't see why our deep interest must necessarily be such a negative thing. Perhaps it is a pride we take in presenting ourselves respectfully for the situation and others who must see us, or the small satisfaction, kept to oneself, that details of one's clothing are absolutely perfect, even if not noticeable to the observer. Or it is self-expression. Or all? I imagine it is different for every person answering the question.