I think it's because it's much harder to shop for groceries in the US. I mean: in Europe you have a small grocery store around the corner, in the US, with single-use zoning, your closest grocery store is probably 3 miles away. So, in Europe you shop more often, in small quantities, in the US you stock your fridge with 2-3 weeks worth of food, just to avoid the hassle.
Not my experience. Canada here but still. You do see these fridges. Have a few friends that have them. I think it's more the 'prestige', marketing and "bling"-ness. "Best" with integrated ice maker and cooled water dispenser.
Personally I don't get it. We have a standard freezer on top fridge, though the size is definitely bigger than what I'm used to from Europe and we have a chest freezer in the garage. That's not uncommon for families in Europe either tho. More of a flat vs house living situation thing.
As a student (living in a flat in Europe) I always joked that I'd go to the 'basement' to my freezer when I wanted a Pizza. I didn't have a basement actually but the parking garage, which was basically shared with (tho sectioned off) a supermarket's parking garage and the supermarket.
I mean, this makes a sort of sense, but on the other hand, I've seen the side-by-side type in San Francisco, where there are supermarkets all over the place. And I don't think they're actually dramatically higher capacity, anyway. I think this one is largely down to cultural preference.
While they're certainly available in Europe, side-by-side fridge-freezers (as opposed to fridge-over-freezer models) are more popular in the US. I don't get the appeal personally, but in any case, both configurations are available in both places.