Because, the ability to go places by water destroys the nice, orderly, slow, gradual, Replacement racial superiority model for the human settlement of the Earth. It may also speak to a deep-seated insecurity in the home of Chuck Darwin (he's a distant cousin, btw) about the fallacy of insularity. Everyone in Britain is descended from someone who arrived by boat, IOW, isolationist dogma (in the US, it takes the form of Clovis-First-and-Only) is invalid.
It fits... Something else I have considered was that since anthropology and archaeology was born of the English like you mention, their narrative was still controlled and limited by the Church so there is a deep set and lasting religious assumption that everyone before them were all just ignorant pagan brutes without the intelligence to be anymore than just grunting animals. This is still believed by many of blind faith even now.