I wonder if this difference is always why Americans don't put butter on sandwiches or consider just bread and butter a completely normal snack. If it doesn't taste like anything I can see why it wouldn't make sense.
Butter is a key component of a quintessential American sandwich - the grilled cheese. Also, bread and butter is a midwestern dinner table staple, and buttered crackers are things I snack on, I put them out for dinner parties. Though, older people use margarine for "health reasons".
If I want tanginess on a sandwich, I reach for mustard.
American butter mostly tastes like salt and hardened milkfat … which is pretty ho-hum.
I suspect most Americans use butter for the salinity rather than the flavor.
I further suspect that people butter their grilled cheese not for flavor, but for crisping the bread and building a barrier against the grease released by the melting cheese.
I didn't find out you could make a grilled cheese with mayo until a few years ago. My husband still won't even try it. In my region, I doubt that most people would be comfortable using mayo instead of butter.
You also have to consider the fact that many households' standard loaf on hand is white sandwich bread. There will always be a place in my heart for the squishy comfort of a slice of our locally-made white bread, but the fanciest butter wouldn't make it a delicacy. Cheap store brand butter on a discount grocery store "baguette" is a much better snack. Then there's butter on saltines to go with a stew or pea soup. The taste of my childhood.
As has been talked to death, the quantity distribution of bread in the US is atrocious. It was certainly my experience when I visited the US. There is a hilariously culturally significant post in /r/Australia where an American tourist talks almost erotically about a loaf of bread they bought here once, and it’s later discovered that it was just cheap, standard, supermarket bread.
We were also heavily conditioned through the 80s and early 90s to not use butter for health reasons. Many homes stopped using lard and animal fats entirely in favor of artificially flavored plant-based butter substitutes that melted poorly and tasted horribly.
I'd say Americans generally expect butter to be used in situations where it's intended to melt. So we use butter on hot toast, hot waffles, baked potatoes, hot dinner rolls, and so forth.
On sandwiches we use mayonnaise since the sandwich is cold, and the eggy flavor of mayo is really tasty, and cold mayo is easy to spread but cold butter is not -- if you used regular sandwich bread it would most likely tear.
Plain room-temp bread and butter we don't eat much of, but that's because sandwich bread isn't usually very good by itself, not because of the quality of the butter.
I think the butter-with-hot-food thing might also be because we usually keep butter in the fridge. I'm aware some people keep butter at room temperature so it's spreadable, but that's never been popular in the US.
Butter is a key component of a quintessential American sandwich - the grilled cheese. Also, bread and butter is a midwestern dinner table staple, and buttered crackers are things I snack on, I put them out for dinner parties. Though, older people use margarine for "health reasons".
If I want tanginess on a sandwich, I reach for mustard.