I don't think that the amount of butterfat in American butter has anything to do with why we don't put butter on sandwiches. We buy Irish butter for its superior taste, and I still wouldn't put it on a sandwich. It has more to do with our sandwiches. You don't encounter soggy fillings to the extent you do in the UK, for one thing. Also where butter might be needed to prevent sogginess, as in a cucumber sandwich, we would be more likely to use cream cheese.
Also as for cheese, there is lots of good and even excellent cheese in the US, but the thing called "American cheese" is not it. But find an aged New York state cheddar, and you have the food of gods.
Some Americans do put butter on sandwiches, but they tend to be Yankees from New England. I married one, but as he now lives in the South with me, he uses mayonnaise and/or mustard as is right and proper.