They don't, really. They just tell you that because either, 1. they're out of your league socially, personality-wise, intellectually, or, as TheBooBooKitty so subtly underlines, physically, or 2. they sense you're just settling for them until someone "normal" shows an interest, and would be inclined to refer to them as fat, ugly, or both. If a normal guy (i.e., someone who gives the appearance of having standards beyond body type and how many layers of makeup they're wearing) were to approach them, there's a definite possibility that they wouldn't have a boyfriend.

If you make it apparent that all you're interested in is a pretty face, you'll likely be a bore to all women you attempt to pick up. And, contrary to some peoples' belief, most "fat/ugly" women are not desperate enough to go home with any shallow loser who shows a passing interest, no matter what suspect allusions he might make to the sexual talents of women of proportion.

Every girl on this planet does not have a boyfriend. However, it's probably safe to assume that they would all appreciate some respect. If you're having problems meeting single girls, try approaching women as people, rather than as ornaments or sexual conquests. 'Cause, hell, you're probably no Ewan MacGregor, either, and you wouldn't want that minor detail to put you out of the game. You expect to woo her with your wit and charm, right? Suprisingly enough, given the chance, she may well have the ability to display those same characteristics.. if you can get past her sub-par supermodel potential.