I'm an American living in Chicagoland, the 3rd largest metropolitan area in the country. I can drive for over 900 miles in any direction and everyone still speaks English, though some jerks Quebec wouldn't admit it at gunpoint. Even foreign born immigrants who never took a class can usually manage enough pidgin English to get by, even if it's just a hundred words or less.

America also has the largest English speaking pop globally. More than the next 3 countries combined. England is 5th on that list.

America had been the dominant global source of money, technology and education since WW2. Though it's in it's slow decline arc, good luck explaining that to uncritical people who were spoon fed the doctrine of American Exceptionalism since infancy.

Why do many, if not most Americans assume everybody else speaks English? Why the hell wouldn't we?

Europeans and Brits go on vacation abroad and it's usually elsewhere in Europe. Huge swathes of the world had to learn English when the British Empire was large and in charge. The US picked up where the Brits left off. It's been that way for literally centuries.

There's a dozen or two languages they might need to be at least passingly familiar with, on top of English which is needed for business, industry, education, flying/air traffic control, and gawd knows what else. It's the lingua franca of the modern age, and if they have no other language in common, two people with a passing knowledge of English can communicate at a basic level.

Americans go on vacation and travel three time zones over without leaving the continental US. A small town is basically the same in New York and California. You can go anywhere in America or Canada (or most major foreign cities) and find at least a few things you know and are familiar with, from chains like Hilton and McDonald's to independently owned clones like Motels and Diners. Our country, a third of the continent (more or less) is larger than all of continental Europe.

So yeah, that's why. Don't worry, the planet will probably kill us off within a generation or two, assuming the Orange Idiot or some foreign despot like Putin decides to end it all early with nuclear hellfire. So if it's a problem for you, at least it's a problem that will solve itself.