I think the fact that people talk about a "full English breakfast" and generally know the gist means it's less broad than the concept of an "American breakfast" which will be vastly different depending on what part of the country you're in. I went two paragraphs deep without mentioning bagels, which are the dominant breakfast option in much of the northeast. My wife is from Arizona, so she basically makes tacos with green chiles for breakfast.

To me, the difference boils down to how people react to substitutions. If you say you don't like part of the breakfast offerings in the states, someone is just going to suggest one of the myriad other things we typically eat at breakfast. When I see people suggest they don't like part of a full English breakfast, the discussion is always about how they just don't appreciate what is being offered and how congealed blood patties are actually great and beans should definitely be slopped on a plate to soak all your other food. If you don't like biscuits and gravy, get one of the 20 other breakfast options that count as regional delicacies in the South. Nobody's gonna judge.