Just because we Brits dont like walking around with perfect ( fake ) looking bleached white teeth...
But yeah, we do live on a small island and the royals especially are badly inbred- my half-hearted point though was that ~250 million Americans are descended from just a few tens of thousands original settlers; thankfully alot of immigration has increased the gene pool somewhat, but we brits have our distant ancestors coming from all over europe & asia and farther afield...
Its still unsettling though all the same...