With the return trip time + parking, you can see how it's much more sensible to plan your shopping ahead of time to optimize for the fewest amount of trips possible.
Even if you currently live in a borderline walkable area, there's a solid chance you grew up in an area where driving to the store was a norm and thus contributes to the decision to walk vs drive.
However, even when patiently explaining this difference, it is a mental leap too far to consider any change. Not only is it the physical design of the stores and car-only infrastructure, a lot of it has to do with package sizing and pricing structure, as another poster pointed out; smaller quantities get massive markups in US stores, for no good reason other than once they've got you in a store, you're fairly captive and they want to extract the maximum amount of money from you so that you don't end up elsewhere.
Until people experience it, and realize that having a five person family is no challenge at all for this style of life, it's hard to give them the picture.
As I mentioned in another comment, I had the opportunity to live for a year in a very walkable neighborhood, with an excellent grocery store. It was convenient, it was small, the food was high quality, checkout was lightning fast. I found myself going there nearly every day and it was wonderful.
But once I needed to move back to a more car-centered city, the idea of doing this became once again unthinkable. Far too big of a chore to do daily.
They literally will not believe that it is possible to but a days worth of groceries every day, and get fresh, quality food.
But the more people saying that it can be done, and is a superior way to shop, the more likely we will convince them finally.