In some areas, such as SoCal, there was a lot of interchange between the custom- and race car communities, the peer pressure to show up with something more finished than the next guy was strong.

The postwar economy was booming, so racers could generally afford those niceties.

Hot Rod, Rod & Custom, Pop Hot Rod and the many iterations of same spread the trends rather quickly - what we would call 'viral' today.

I'd likewise imagine other areas where there was not ready access to painters, platers and upholsterers the finish work would be a lot more spartan.