Are you kidding?
It's legal only because the FAA is worried that people will drive over flying if they make them buy infants a carseat.
But there's no argument that it's far safer for kids to be in a carseat than being held. Turbulence does happen.
Agreed that it is probably safer. So is buying car A over car B potentially. I should hope that the reason it is legal is because it is as safe, all else equal, in a lap as in a seat if the flight goes well. Even with turbulences it should be safe otherwise I can't see how this would have ever been approved.
Them being worried cannot be the only reason it is legal. They would have been sued left and right for mega millions if there was actually a real issue. (I would think)
Best case, Id love to buy my children a seat every time I travel. It is easier on me and probably somewhat safer. But with prices where they are it is just very difficult to do. Sadly.