For the same reason that they can’t muck out large cattle pens by hand, or chop the firewood, or pile 60-bale loads of hay on wagons all day every day during hay-season. The heavy farm labour is for the men who don’t get pregnant or suffer from menstrual cramps. It is the men’s duty to bring home the bread; it is the women’s duty to put that bread on the table.
I realize there may be some women who are willing and able to do these jobs but the Amish believe in segregation of labour. They believe, rightly or wrongly, that too much interaction between men and women causes affairs and premarital pregnancies.
However, the question is not “Should Amish women be allowed to help with barn raising?” The question is “Why can’t Amish women participate in barn raising?” Those are some of the practical reasons for the Amish being a strongly patriarchal society. The religious reason is that the man is ordained of God to be the head of the home. See; ; .
Some patriarchal proponents also argue from the story in Genesis in which Eve was deceived by the snake and gave Adam the forbidden fruit. It was Eve who was deceived, they argue, not Adam and therefore males should be the head of the home; this proves—they believe—that women are more prone to deception and inferior thought than men. That Adam didn’t have the backbone to take responsibility for his own actions and to refuse the fruit his wife offered (even though God held him responsible for this) none of these proponents point out.
That’s the best answer I know to give. Generally, the women are so busy—and happy—feeding the men at a barnraising and washing up afterward that they have no time to help with the actual carpentry. In their spare moments they like to admire their men working up there on the newly raised frame against the sky in the same way that other women like to admire their football player men in action.
Next question: Why can’t American women play on major league football teams?