Because as soon as you hear the word Dutchman -- which is an archaic word, nobody calls Dutch people "Dutchmen" anymore -- but as soon as you hear Dutchman, you get this picture of the kid with his finger in the dike, preventing the thing from falling and flooding Holland, and so on.
因为一旦你听到Dutchman这个词—— 那是一个不再被用来称呼荷兰人的古老词汇—— 当你听到这个词， 你会联想到有一个小孩将手指放在堤坝中 为了防止堤坝脱落并淹没荷兰而之类的画面。