I thought that all those panel lines were left off of promotional models because those lines are air brushed out in most old car brochures and advertising of the period. I guess auto makers found the lines distracting from the overall flow of the vehicle. I guessed that the panel lines missing on kits was just a carry over from the promo origin of them.
I have no explanation why they'd be missing on modern kits, especially ones geared at our picky adult market.
Quite probable that the companies making promotional model cars were having to work off of automaker-provided pictures of approved styling clays, which for the most part had only the essential panel lines modeled into them (hood, trunk lid, doors, gas filler door).