Or because the U.S. is on the whole right wing by European standards - and because there is no tidy division.
Not yet, and maybe never because nobody will try to sort it all out, but there's nothing in principle preventing it. Simply allowing for the left or right overall differences is already a help toward sorting it out.
For instance, whatever the Tory leadership really think about the NHS they certainly aren't prepared to say they want to scrap it. But you would be seen as a "leftist" in the U.S. to suggest any sort of equivalent there.
So we allow for the left-shift in the UK and that explains why what here is far left is only somewhat left there. That appears to be a consistent pattern.