Is it? Where are you getting your facts from?
This sounds like one of those statements that you hear, with no genuine and up-to-date research data to substantiate it.
Incest is a very taboo subject and the reporting of it is low. The likelihood is that it's something that's gone on since - well - since human beings have been on earth!
Maybe figures are showing an increase because, in our more liberal society, people are more willing to report it?
As for incest being high specifically in America - I doubt very much that this is the case.