Do you mean 2-3 per person or 2-3 per household?
I'd have a hard time believing we have 2-3 per person (No I didn't research it), but 2-3 per household is not surprising at all.
There's a few things that play into this...
Our public transit SUCKS.
Conflicting work schedules, can't carpool with spouse.
I don't think the number of children matters much, until it comes in to what you need to do to support them. I have 3 kids, my whole family is comfortable in my truck and we drive it everywhere. We have something every evening, Mon - Thurs, but fortunately we avoid conflicts so only one car needs to be used.
I think in the US we also spend more time in transit than in EU. Personally, I have a 30-40 min drive to work, while it's a little long, I don't think it's uncommon. I think a large percentage of people have a 20-30 minute drive. If one parent is off to work and the other parent needs to go somewhere, you need that second car in the household. And this comes back to our public transit, it sucks as soon as you leave a major city.