Overwhelming majority of Americans live less than 20 minutes drive from a proper grocery story (i.e. not a gas station convenience store), and most probably live 10 minutes drive. In my metropolitan area of 4 million, I can't think of a place that's more than 10 minutes from a large supermarket.
Americans don't do grocery shopping very often simply because it's such a waste of time: there are 5 large supermarkets (two Safeways, one QFC, one Whole Foods and one local chain) 5 minutes drive from my home, and there's also Costco 10 minutes drive. Despite that, I go grocery shopping once a week, because it usually takes an hour. Doing this more often would replace one hour-long trip with three 45 minutes trip. Since the drive is only 5 minutes, living literally next to them wouldn't change my habits much.
Also, let me assure you that most Europeans don't live in 5 minutes walking distance from grocery stores, so the travel time here is not what makes the habits different.
And even though we have a selection 2 to 3 stores in 20 minutes drive ( one we could walk to if we chose to forego convenience of one trip a week ), I remember living in locations where access was not as simple.
From my experience, Americans often do grocery once every couple of weeks, because the super market is usually a non-trivial drive away.
I can see how a lot of their foods developed to accommodate that shopping cycle, and forced preservatives that make things last those 2-3 weeks while compromising on freshness and taste.