Why not engineer a design where in an emergency, the seats turn into a cocoon capable of bouncing on impact and encase the passenger in such a way that impact with the ground will not harm him. of course the seat would have to be built of some new material maybe graphene that is 100's of times stronger than steel and would have to have points of contact with the passenger's body to prevent jarring the body's organs etc. Now cost maybe prohibitive but I think engineers can devise such a seat mechanism that may be light enough but yet strong enough to do the job. is it possible?
In a word, cost.
Weight equals cost.
How much is this seat going to weigh?
How many paying passengers will not be able to fly because of the added weight of the seats (and the associated cocoon-deploying equipment)?
How much more fuel will the airplane burn to carry all this extra weight?
Design equals cost
This super-strong material doesn't exist yet. How much will it cost to invent? Who will pay to develop this material?
How much does the material itself cost? If it's so wonderful why don't we make the whole plane out of it, as every stand-up comic suggests?
How much will the research to perfect and certify the deployment technology cost?
Passengers panic when the oxygen masks drop, we can't have the cocoon seats deploying unless there's an actual emergency.
What if there is an actual emergency and the seats don't work?
Deployment equals cost
How do passengers evacuate the aircraft if they're trapped in cocoons? Do emergency crews have to cut into the structure of the aircraft to get them out? Now the airplane is destroyed. New airplanes cost money.
What if the aircraft ditches in the Hudson river? Do all the passengers drown? Maybe we should make all the seats eject out of the aircraft. Guess what? Destroyed aircraft still cost lots of money and now you've got the extra weight of the ejection devices.