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.
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.
To YOU it is (and I agree with you). But people make it with oil or mayo as the fat. I doubt most people think of butter as a required ingredient.
[1]: https://www.reddit.com/r/australia/comments/f9avca/i_ate_bre...
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.