Earlier quoted context omitted.
It is not easy to fit three car seats in the back of seat of the average vehicle in the US. I bet this is not a hinderance in the rest of the world because many of those other places have either: 1. a lack of child seat laws 2. effective public transportation. It's not like Europe is full of A-segment cars with 3 car seats in the back.
The median vehicle in the US is fully capable of seating three child safe seats. Yes, if you get a Honda Civic you're going to struggle. But sales of sedans are absolutely swamped by SUVs and trucks. The best selling car in America by a long shot is the F-series pickup truck, with about 900,000 units sold. You can easily fit three child seats in it[0] Oh, the number two and number three best selling vehicles are the…
But yes, even many mid-size cars and SUVs are difficult to fit three seats in.
https://www.consumerreports.org/car-seats/how-to-fit-child-c...
> we assess a vehicle’s ability to securely and easily accommodate three child restraints in the second row. Our test results have shown that unless you’re driving a full-sized crew-cab pickup truck or an Audi Q7, the installation is going to be difficult, if it's even possible.
(It is kind of funny that your source shows a picture of a single cab 97-03 F150 in regards to fitting child seats three across... :D )