America is too big to understand. Even though it has been mapped and analysed to the most minute level of detail, no mind can take in the size of the sprawl between its cities. We can't block out the signs in our heads that read Here Be Dragons.
We can understand England, though. We can cross it in less than a day. We know the names of its smallest towns. We can imagine the roads that link them. If there were dragons in the fields, we would have seen them.
There's no space to add anything impossible in England’s landscapes. Lewis and Tolkien and Pratchett and Peake had to build new Englands in impossible constructed realities. And yes, plenty of Americans have created imaginary worlds too. But they don't have to. There was still room on the map.
Twin Peaks and Pawnee and Gotham City and Metropolis and Miskatonic University can all be reached by road. Look at the impossible towns in John Crowley's Little Big, or Jonathan Carroll's The Land of Laughs, and they fit comfortably into the landscape. You don't need to work at believing that there's space for them.
Try to do this in the UK, and you need to persuade people that there's room for something new. Alice's Wonderland, or any of Gaiman's hidden places, have narrow doors that few know how to pass. The towns in The League of Gentlemen, or Hot Fuzz, or The Wicker Man are protected: outsiders are repelled. It's not like America, where you can just walk in.
(There's an annoying counter-argument to this divide: 19th century realist fiction, which is full of places like Middlemarch and Casterbridge and Barchester and Coketown. Maybe imaginary places are easier to accept on a small map when they aren't impossible. Or maybe England was bigger before cars).
But in America, we’re happy to accept that there might be dragons. Not kept out of sight. Just in a field we haven’t visited yet.