Want to do a big shopping trip? Have more than 1 child you need to move around the city? Want to do more than 1 specific errand in the same afternoon? Without a car, all of this becomes a huge pain in the ass even in New York or Boston, which have extensive networks of subways and buses, let alone anywhere else.
And now let's say you want to get out of the city for the weekend. Do you rent a car? That's way more expensive in the long run compared to owning.
We would need to fundamentally redesign our cities/towns and transit systems for Americans to be able to give up their cars.