On big American roads it's easy to own big cars. Given that people have to drive a lot because it's so spread out, it's very convenient to be able to put lots of stuff and/or people in a vehicle. I theorise that your car is like another room in your house.

I know my wife likes storing things in the boot of our car and I'm not even American. It means they're always conveniently there - chairs for sitting in the park, shopping bags, groceries that she's going to take to a party or bought for someone else, kids sports equipment.