That’s a gross oversimplification. You’ve got rear-facing seats for babies, front-facing seats with five-point harnesses for toddlers, booster seats with backs for the next age / size grouping and then backless boosters for the age grouping after that. All with straps and buckles that differ and must be adjusted to the kid’s size. All with different safety ratings and recall statuses.
My girlfriend who’s a pediatrician says that something like 90% of car seats are installed incorrectly. I know where I live the fire department will check your installation for free as they’d rather expend time on that than on resuscitating kids suffering trauma that wouldn’t have been that bad had their car seat been installed properly.
My brother tells me it costs as much to rent a car seat as it does to rent a car. (ie renting a car with a car seat doubles the cost vs just renting the car). While that’s fine for a one week a year vacation… it might not be fine for daily drives to daycare. And you really don’t know how clean or safe or old the rental car seat is.
If it’s old the plastic might be brittle and less safe. You’re not realistically going to check for recalls every time you go somewhere by car with a kid if that’s a frequent occurrence. And, while I gotta say that my niece’s car seat is hardly a model of cleanliness… at least they know that it’s HER cracker crumbs, HER snot, HER puke or pee stains that you can still see even though they washed the cover three times after the puke/pee happened. The ick factor goes up considerably when it’s crumbs/snot/stains of unknown origin.
Nah, for parents who are driving with their young kids on a regular basis, the convenience of having their own vehicle is going to remain tremendous.