- travel with many kids (nope, physically can't carry 4 car seats plus luggage)
- using a taxi, e.g. to go see a movie (nope, can't carry a car seat into the theater)
- carpooling with other families (I'll drive them, you pick up? Nope, we'd have to shuffle car seats around.)
- rides with grandparents or other family members (sorry, we'd have to deliver the car seat to them first)
- splitting kids between two vehicles for errands (let's spend 10m wrestling car seats from one car to the other first)
The whole texture of independent childhood is altered by car seat mandates! Everything gets filtered through "is there a car seat available?". If you haven't experienced this, it's hard to describe - and I think it's absolutely a case where tradeoffs like "how will this affect quality of life?" are completely overridden because "well, if it just saves one life..."