In his new book Encounter, Milan Kundera writes:
I was rereading One Hundred Years of Solitude when a strange idea occurred to me: most protagonists of great novels do not have children. Scarcely 1 percent of the world's population are childless, but at least 50 percent of the great literary characters exit the book without having reproduced. Neither Pantagruel, nor Panurge, nor Quixote have any progeny. [...] and Kafka's protagonists, except for the very young Karl Rossmann, who did impregnate a maidservant, but that is the very reason -- to erase the infant from his life -- that he flees to America and the novel can be born. This infertility is not due to a conscious purpose of the novelists; it is the spirit of the arc of the novel (or its subconscious) that spurns procreation.
Toss in Melville and Conrad while you're at it. What I find striking, however, is that contemporary writers seem more likely to give their protagonists children (Roth, Franzen, Updike, for a start, plus the rise of female authors helps this trend). And that is precisely at a time when more people are having no children at all. The decline of the heroic ideal in literature, and the decline of the journey of adventure, seem to be stronger forces in predicting fictional family size.
I find Cowen's comments at the end not really puzzling in themselves, just that he passed on the quote from the book as self-evidently interesting. And maybe there is more after that quote from the book that clears it up, but my reaction to the quote is, "Because Western novels are about people who do shit." In fact Cowen acknowledges that when explaining why there are now fewer childless protagonists: the decline of the novelistic "journey of adventure."
The quote is a good discussion prompt, but not so much because it is so mysterious why protagonists are childless as because there must be like a dozen really good reasons. First in my mind, the Western protagonist is by definition and intent of creation, an exceptional adult, who gets to do interesting things (things normal adults only fantasize about) because of 1)ability, 2) immediate circumstance, 3) larger circumstance (not being hindered absolutely). Parenting hinders absolutely. There are disabilities that might make a novel more interesting, like if the protagonist is narcoleptic, but if he or she has kin, then the whole novel would just be 250 pages of other people yelling "Please! Tammy, you really shouldn't be doing all this novelly shit and need to take your kids to the pool!" Even if the old middle class which birthed the genre of novels could in practice outsource their real-life child-rearing without reprobation, the identity of parenthood interrupts the reader's conception of the protagonist's selfness. It strips them of that nervous intellectual self-concerned energy whereby they will usher the reader onto an absurd testing ground for unpractical ideas.
Second, the Western Protagonist is driven by conflict. Romance and Warfare, for example, are conflicts whose structure and shibboleths are really genetically similar. Other traditional fictional/nonfictional (doesn't make much difference in Western writing because we use the same conventions of narrative for both) other traditional conflicts have large-percentage DNA matches to those. Child-rearing is not just a different animal but a different kingdom, say the plant kingdom. Child-rearing can have drama and triumph, but it is not self-centered. Romance and war build up the ego of the Western protagonist; those make the Western protagonist more heroic by result of conflict. Parenting is not a struggle that makes the ego stronger, it is a process of self-subsumation.
And there are probably more reasons for the childless protagonist, that can be added to that list. But the idea that novels have been devoid of parents as some type of commentary on procreation is silly: novels are devoid of parents because they are incapable of commenting seriously on procreation within the bounds of fictional convention.
Not that this is as it should be: I think you can make a good fiction about a parent and generally there should be more fiction that takes place in a conflict-null universe. I find the prevalence of the Western convention of conflict-centric narrative in fiction and non-fiction a little boring now that I am no longer 16 years old, generally.