I don't think those people are as insecure as a lot of people think they are. I think, instead, that they often think more highly of themselves than they really ought to. :/ My theory is that they were kids who grew up without ever having a parent tell them, "Look. You're wonderful, and you're really loved; but no matter how wonderful and loved you are, there are a lot of other people in the world who are as wonderful and loved as you are." I'm not AT ALL in favor of pushing down a child's self-esteem, but - really - there's a way to nurture self-esteem while also putting things in perspective.
So, I think there's there's that (above), and I think, too, some people have a "measure of a man" that's based on superficial, isolated, things; and if they excel in one of those things they get to feel superior overall. Someone who values in others what is important in all human beings will recognize that he's not better than anyone else.
Or else, you're right (Pearldiver). They're insecure about the stuff that involves being a person in general, so they latch onto some superficial thing (like who has the most money or who is the best looking, etc.) and use that at their measure (because they can't use what really matter as their measure of equality and worth as human beings).