We have many small cities with nothing between them, very little intercity rail services, and an easy source of cheap second hand cars. I'm surprised we are first but I would have put money on top 10.
I'd say it's very similar in Poland. A lot of small cities and villages, barely connected by roads and with little to no rails. Public transport is good but only in big cities. In those smaller ones, you're lucky if there's more than on bus per line per hour.
This is my firsthand experience, as I'm from one of those. 15 years ago there was a bus every 15-20 minutes. Nowadays there's one per hour, sometimes less.
I'm honestly surprised that the US is in second, off the top of my head I only know one person that doesn't own a car but her boyfriend has one that they share.
Something that brings that average up, in my view, is the number of classic cars we have on the road. These are vehicles that don't do a lot of driving, but are technically in use.
There's also a lot of people, especially trades, that have company vehicles that they can't use for personal use, so they also have their own vehicle.