My theory...
Because it easier than trying to get a good match with the outside of the car when you paint it. Lower production cars, like the Dino that I used to have, do it from the factory. I suppose that is because it is easier to just spray the compartment, and sometimes the trunk, black and wait for the customer to specify an exterior color.
And the infant's name is......? I am expecting my first grandchild in March and I am a certified, licensed infant nut case. Life will be over the top when Dixie Rose Blue arrives! LOL LOL Jack