The people who design car seats these days know that their customers would rather spend an hour rethreading/reinstalling their car seat every six months than spend over $200 on the seat. But if there were a large fleet of shared cars on the road, and if some entrepreneur came out with a $1000 car seat that could be readjusted in seconds for a child of any size, then the owners of the fleet would have an incentive to loan out those seats along with the cars.