Other countries have national dishes, even if it means a variety of regional cuisines that have strong similarities. Think Mexican tortillas and beans, Italian pasta, Japanese rice and fish. But the United States doesn't seem to have a national cuisine; there is southern food, and regional specialties, but nothing that ties it all together, except maybe our tendency to grill meat and boil vegetables. It seems that the only real American food culture is the melting pot, the tendency to fuse unrelated flavors and techniques from other cuisines.
I first thought about this when I saw fahita pizza on a menu; it used barbecue sauce instead of tomato sauce and was topped with steak or chicken and sauteed onions and green peppers. I've also seen burritos made with Indian ingredients and hamburgers with wasabi. I suppose one way to look at these combinations, some tasty, some jarring, is as a bastardization of the ancient and noble cuisines; Americans are uniquely talented at ruining every food they touch. I prefer to think of it as a sign of creativity and openness to other cultures; Americans accept good things anywhere they find them. (Of course, part of this is because I tend to use flavors I like from other cuisines in my standard cooking, using curry powder on a steak or chili powder in soups.) And only the scale of the borrowing is new; the exotic combinations are simply an extreme version of the traditional response to new food items, absorbing them into the prevailing methods of making meals. Witness Italian use of tomatoes and Indian us of peppers from the Americas, as only two examples.
American mix and match more than dishes - we mix and match cuisines, eating from a wide range of ethnic foods on a regular basis. As an Iranian-American character in Digging to America, by Anne Tyler, thought, "By now, she was aware that Americans thought recipes were a matter of creative invention. They could serve a different meal every day for a year without repeating themselves - Italian-American one day and Tex-Mex the next and Asian fusion the next - and it always surprised them that other countries ate such a predictable menu."