While I definitely think there’s truth in what you say, I don’t think it’s the real reason. The posture car seats try to put you in is just not good sometimes. If the seat itself lets you sit up straight, the headrest juts out, or the headrest is okay but the seat is curved into a bowl. It’s comfy for lazy sitting, which is what most people will want to settle into, but if you try to be mindful of your posture, you’re doing it without real support from your seat.