^ Parent w/ a <1 yr old. Car seats aren't used on planes because a car seat would take up a seat, which costs money. Kids under 2 yrs are free if they sit on your lap at most/all US airlines, so parents would just swap lap-sitting for their kid(s).

Not sure about trains.

I assume that if/when cars are as safe as a plane/train, adults wouldn't even need seat belts but you'd still need a carseat for an infant/toddler because it also stabilizes the child's position. Kids <6mo have no neck control. Kids <10mo can't reliably sit up for long durations. An autodriving car might be safe, but it'd probably still need to make emergency stops / turns. That'd be real bad if there were no carseats.

Car seats are now encouraged for planes for < 18 months. The worry isn't crashes per say but jerks and sudden stops on take off and landing, and turbulence.

https://www.faa.gov/passengers/fly_children/

It's a PITA to use due to the huge variety of seat dimensions and car seat sizes and no standardized latching system for plane seats.