A flight is infinitely more traumatic than a car ride, unless the car does a few hundred miles an hour & bounces up into the sky then comes bouncing back down again. I don't know how you can even question why a flight would be more traumatic to a baby than a car journey.
i think you've been unlucky with your flights.
once in the air, in my experience, its far smoother than a car, there are no corners for a start. I remember flying as a child, i was totally disappointed. I think i was expecting to feel the wind in my face or something but its no more exciting than being on a bus.
As for take off and landing, thats nowhere near traumatic [ unless you've given birth to some highly strung fragile little freak that should be on valium anyway ]
Children arent that easily traumatised IMO, they fall down stairs, thier parents and nieghbours do DIY, they are exposed to music TV, traffic, they get ill, they go through hot summers, cold winters....blah blah blah
and of course they also experience all this kinda stuff in the womb, in itself a noisy place.
all this intense noise and movement, thier brains soak it up, it is not harmed by it.
that in mind i really dont think a few seconds of being pulled into thier seat on take off and a gentle bump on landing hurts babies.
oh and the issue is, for me, one of consent.
i have seen my eldest child on valium [ she got it from A&E...long story ] when my ex brought her back falling arse over tit and hearing things i was fucking livid, it really messed her up
the idea of doing that to a baby kinda makes me shudder