For a bit I had 5 cars with 4 drivers (7 vehicles if you count the 2 motorcycles), but am back down to 4 cars and 1 motorcycle for 4 drivers.
I have a 21 year old F150 and a motorcycle.
Wife has a 5 year old Hyundai SUV.
Each kid has their own car (13 and 11 year old Scion's).
I had a 5th car (19 year old Hyundai) for a bit when my son bought his own car (I kept this one since I bought).
All are paid off (and have been for years).
Was nice having the "extra" vehicle.
$40/month for insurance and gas was $0-50/month depending on how much it was driven.
I let a friend borrow it for a week or so when they had car issues; daughter used it when her A/C went out; sister used it when she came into town; I would use it on longer trips (almost twice the MPG of my truck).
Ended up giving it to a nephew in medical school who is recently married and a new kid and needed a second vehicle.
I could probably get away with getting rid of my truck and use just my motorcycles for daily use and borrow my wife's vehicle when I needed it, but my truck cost me about $1,300 a year for insurance, gas, maintenance and repairs.
And I like my truck.
As many have said, in the suburbs and rural areas, a vehicle for each driver is almost a must.
No such thing as "mass transit" in many areas of the US.
Even around me (8 miles to Houston City limits), it is 4 miles to a bus station that can get me into Houston's transit system.
And if I actually want to get anywhere in a timely fashion, forget it.
Even something you would think of as simple as getting to the Texas Medical Center is anywhere from 90 minutes (taking a bus then transferring to the light rail) to 2 hours (using a different bus station about 8 miles away from the first one, but is direct to the Medical Center). This is a 33 mile trip. Even in the worst traffic, it takes me 90 minutes to drive, and good day is around 40 minutes (again, the best mass transit can do is about 90, and that is only on weekdays, weekends have fewer trips, so would have to wait longer for a bus).
I think this is also a part of the issue so many have with mass transit, it is on their (mass transits) schedule, not yours. The closer to the city centers you get, the more convenient it becomes, but further out, not so much.
As for size comparisons, it is not just Europeans that don't understand how vast the US is.
I work with a lot of Philippino's and when they have family that visits, they seem to want to visit New York, Florida, The Grand Canyon, California, any other points in between, all while driving from the Texas Gulf coast; they are here for maybe 2 weeks.