... because swagger makes you try things a modest person would never attempt.
My daughter plays soccer. She plays defense and does a good job of it, but whenever she gets control of the ball, she gets rid of it as fast as she can, either by clearing it or by passing it away. A touch or two and it's gone. She plays like she thinks she is the least deserving person on the field. She's only half right. In practice, she's got good ball control skills, but in games, her skills are uncetain. Girls with swagger don't have this problem.
What's missing is the conviction that she can dribble up field herself because she knows she can get by the players on the other team. Girls who think that way try it and trying things is the key to learning how to do them. At first they fail quite a bit, but their innate self-confidence keeps them at it until they've got it down in game situations. If you're modest and keep passing the ball off to players who you think are better than you, you never try and so you never become a top player.
Maybe all of those NFL players who dance and strut are in the league because they danced and strutted.