You could attribute that to humility or good manners, but you could also attribute it to the fact that their persona is good enough of a signal that they don't need to add another signal.
This is especially true of traditional nobility and the super-rich. It doesn't matter whether Bill Gates wears a $40 watch or a $40K watch because everyone already knows that he's one of the richest guys in the world. Marie Antoinette wore diamonds not because she felt a need to demonstrate her status and power, but because she liked diamonds and happened to have the status and power to get them.