Because “The Farmer in the Dell” is not just a song but a dance. At the beginning, one child, the “farmer,” stands in the center of a circle, and the other children sing and dance around him/her. On the first verse, s/he choses a spouse, and that child leaves the circle to join the farmer at the center. The spouse takes a child on the next verse; on the next the child picks a nurse; shortly after that it’s a barnyard menagerie that lasts long enough to pack the rest of the kids into the center, with the proviso that the last four characters “taken” are the dog, cat, rat, and cheese. The cheese, having no one left to “take,” stands alone, and gets to be the farmer on the next go-round.
When I was a kid there was a lively argument over whether the cheese stands alone or the cheese picks the ketchup and the ketchup stands alone. I like cheese, and I like ketchup, but I don’t want any intermingling of the two.