Why does the author continually use "so it goes"?

"And so it goes" is a refrain that is repeated after deaths in Vonnegut's book. And on a first reading may seem to be a part of an argument that there is nothing that anyone can do to stop death or war. It is important, however, to realize that the book is a satire, that looks critically at the apathetic position that it describes.

The Tralfamadorians accept such a position, and Billy Pilgrim seems to as well, but neither of the aliens, nor Billy Pilgrim are presented as heros or examples for readers to follow.

The hero of the Tralfamadorians, remember, is Charles Darwin. Billy Pilgrim's story of apathy ends in an apocalyptic scene in which civilization is destroyed. The birds don't seem to care but we, as humans, are meant to.

Thus, although, "and so it goes" makes it sound like Billy Pilgrim doesn't care, when we read the phrase as a part of a book of satire, we are meant to read its tone as that of a lament.