The younger children of British aristocrats didn’t inherit titles*, so they eventually intermarried with commoners and faded into the general population. It isn’t so surprising that so many Americans have a dash of royal ancestry. It was different on the continent where all the children and grandchildren of nobles remained noble.
*Well, they had “courtesy titles” like “the honourable” and “lord” or “lady,” but unless they could earn a title of their own, their children would be commoners.