More like the US likes to think it has a lot of leaders who deserve such recognition. On a more realistic level....pretty much every single US leader simply isn't remarkable at all, most certainly so on a 'great leader' basis. Yeah, we've got a whole lot of Churchill-esque personality in there that makes them stand out, but we haven't exactly got people who caused colossal social revolution or raised the nation from the gutter to glory.
Well sure, if you're looking at leaders based on their achievements rather than notability, but assuming you're looking at it on the basis of notability, well, a lot of American presidents are reasonably notable.