The vast majority of the rest of the world simply doesn't use car seats at all. Even here in the US, in my very multinational city, I only see one demographic that consistently uses car seats at all, regardless of economic status.
I think this apply also outside of "the rest of the world". Nobody bothered to explain that an airbag without a seatbelt can be fatal or that in case of emergency braking the only thing stopping your passenger's movement is the windshield.
I wouldn't want a third kid just because I don't want to deal with carseats anymore.
You can buy special car seats that can fit 3 across even in the smallest cars of Europe.
By the time you had your third, would the oldest still even be in a carseat?
But even then, three car seats is doable even in small cars: https://www.jefftk.com/p/three-car-seats