thats because the people who run airlines are smart and they know that people wouldnt fly if they *had* to buy a seat for their infants. also then the accident rate for motor vehicles would increase dramatically from the increased traffic. so even though it is extremely dangerous to hold a child on your lap on a plane, they let you do it. kind of like how we all didnt have car seats when we were little. and a car seat would be a huge help if the plane skidded off a runway. the child would fly out of your arms if you came to a sudden stop, but a car seat securely strapped in to a seat would protect your child. also, in a collision on the ground (or close to the ground) even if you were somehow able to hold on to your child and prevent them from launching across the plane, the force of your body smashing your child against the seat in front of you would be about 1000 times your normal body weight. the child would serve as an airbag for you, basically. a car seat would protect your child in that case as well.
sorry to be all preachy, but my friends husband is an expert in airline safety and accident investigation and ever since he told me about all of this i cannot believe it is legal to allow children on planes without proper restraining devices.
and all that being said, my kids (2yo and 8mo) will be riding on disney transit without car seats. so go figure