They do it so they can look cool, like James Dean.
Many people think the prime down in jackets is eiderdown. While true, there is a super prime down where the heat retention qualities are vastly superior to eiderdown. Of course, this super prime down is quite rare and is only used in the finest in outerwear, for a price.
You can easily spot those who own one of these jackets because the jacket shoulders have a extra insulation gap to hold this super prime down. When observed it makes the jacket wearer appear to have hunched shoulders even though they are not doing that at all.
It’s called hunkerdown.