Not sure how much we need to optimize for this. Plane crashes are rare. I'm happy to risk a baby seat crashing into me if my plane crashes if it means the 99.99987% of the flights are slightly better.
Turbulence that loosens objects is very common.
My guy a car seat will do nothing in a crash.
The math is more that benefits aren't that great and the dangers are pretty big, especially if someone fails to properly secure the carrier and you get a rejected takeoff