Sure, but I think it's a large part of it. It's such a social thing. If you've been successful with women all your life, you'll probably pretty secure, whereas if you haven't been, it's more difficult to be. Good looking guys will have girls fawning over them, that's the way it works, especially when you're young. The boys are on one side of the room, and the girls on the other, none of them are brave enough to talk to the other, so looks play a big role, at least image if not looks.
Anyways, I'd say that in general good looks = more successful past = less insecurity. Of course, that's sort of an overgeneralization, but an observation nevertheless. The less good looking guys who seem to be confident probably are the ones that need alcohol in their systems to achieve that, and even then, they have the danger of bouncing to AFC mode at any second.
From a guy who doesn't drink but talks to a ton of women randomly, I can tell you right now, that most of my friends who are good looking aren't genuinely confident, but they can sure put up a pretty good act when they have alcohol in their systems. Problem is that they are deep-seated AFCs at heart and it comes out often.