The reason is that a military defeat of Britain will bring about the disintegration of the British Empire. This would not be of any benefit to Germany.
“Success is sweet and sweeter if long delayed and gotten through many struggles and defeats.”
“Europe has what we do not have yet, a sense of the mysterious and inexorable limits of life, a sense, in a word, of tragedy. And we have what they sorely need: a sense of life's possibilities.”