This is not a purely German phenomenon - a lot of "English" and "Scottish" folklore comes from the same era or rather later. In our case, it was partly because of the Victorian and Edwardian conviction that all rural English popular culture had to be an indescribably ancient survival of pre-Christian pagan lore, something which is more and more challenged now - for example, the theory that the famously priapic Cerne Abbas Giant was created in the seventeenth century to troll Puritan sex-phobia, and the fact that Herne the Hunter doesn't appear to have any written record prior to the alleged legend's use as a plot point in Shakespeare's The Merry Wives of Windsor.